Add pushover notifications, this should be a super basic MVP

vendor/github.com/gregdel/pushover/README.md

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+pushover
+=========
+
+[![GoDoc](https://godoc.org/github.com/gregdel/pushover?status.svg)](http://godoc.org/github.com/gregdel/pushover)
+[![Build Status](https://travis-ci.org/gregdel/pushover.svg?branch=master)](https://travis-ci.org/gregdel/pushover)
+[![Coverage Status](https://coveralls.io/repos/gregdel/pushover/badge.svg?branch=master&service=github)](https://coveralls.io/github/gregdel/pushover?branch=master)
+[![Go Report Card](https://goreportcard.com/badge/github.com/gregdel/pushover)](https://goreportcard.com/report/github.com/gregdel/pushover)
+
+pushover is a wrapper around the Superblock's Pushover API written in go.
+Based on their [documentation](https://pushover.net/api). It's a convenient way to send notifications from a go program with only a few lines of code.
+
+## Messages
+
+### Send a simple message
+
+Here is a simple example for sending a notification to a recipient. A recipient can be a user or a group. There is no real difference, they both use a notification token.
+
+```go
+package main
+
+import (
+    "log"
+
+    "github.com/gregdel/pushover"
+)
+
+func main() {
+    // Create a new pushover app with a token
+    app := pushover.New("uQiRzpo4DXghDmr9QzzfQu27cmVRsG")
+
+    // Create a new recipient
+    recipient := pushover.NewRecipient("gznej3rKEVAvPUxu9vvNnqpmZpokzF")
+
+    // Create the message to send
+    message := pushover.NewMessage("Hello !")
+
+    // Send the message to the recipient
+    response, err := app.SendMessage(message, recipient)
+    if err != nil {
+        log.Panic(err)
+    }
+
+    // Print the response if you want
+    log.Println(response)
+}
+```
+
+### Send a message with a title
+
+There is a simple way to create a message with a title. Instead of using pushover.NewMessage you can use pushover.NewMessageWithTitle.
+
+```go
+message := pushover.NewMessageWithTitle("My awesome message", "My title")
+```
+
+### Send a fancy message
+
+If you want a more detailed message you can still do it.
+
+```go
+message := &pushover.Message{
+    Message:     "My awesome message",
+    Title:       "My title",
+    Priority:    pushover.PriorityEmergency,
+    URL:         "http://google.com",
+    URLTitle:    "Google",
+    Timestamp:   time.Now().Unix(),
+    Retry:       60 * time.Second,
+    Expire:      time.Hour,
+    DeviceName:  "SuperDevice",
+    CallbackURL: "http://yourapp.com/callback",
+    Sound:       pushover.SoundCosmic,
+}
+```
+
+### Send a message with an attachment
+
+You can send an image attachment along with the message.
+
+```go
+file, err := os.Open("/some/image.png")
+if err != nil {
+  panic(err)
+}
+defer file.Close()
+
+message := pushover.NewMessage("Hello !")
+if err := message.AddAttachment(file); err != nil {
+  panic(err)
+}
+```
+
+## Callbacks and receipts
+
+If you're using an emergency notification you'll have to specify a retry period and an expiration delay. You can get the receipt details using the token in the message response.
+
+
+```go
+...
+response, err := app.SendMessage(message, recipient)
+if err != nil {
+    log.Panic(err)
+}
+
+receiptDetails, err := app.GetReceiptDetails(response.Receipt)
+if err != nil {
+    log.Panic(err)
+}
+
+fmt.Println("Acknowledged status :", receiptDetails.Acknowledged)
+```
+
+You can also cancel an emergency notification before the expiration time.
+
+```go
+response, err := app.CancelEmergencyNotification(response.Receipt)
+if err != nil {
+    log.Panic(err)
+}
+```
+
+## User verification
+
+If you want to validate that the recipient token is valid.
+
+```go
+...
+recipientDetails, err := app.GetRecipientDetails(recipient)
+if err != nil {
+    log.Panic(err)
+}
+
+fmt.Println(recipientDetails)
+```