Send messages to topics for android (#296)

Signed-off-by: Bo-Yi Wu <appleboy.tw@gmail.com>

vendor/github.com/appleboy/go-fcm/README.md

@@ -1,8 +1,8 @@
 # go-fcm
 
-[![GoDoc](https://godoc.org/github.com/edganiukov/fcm?status.svg)](https://godoc.org/github.com/edganiukov/fcm)
-[![Build Status](https://travis-ci.org/edganiukov/fcm.svg?branch=master)](https://travis-ci.org/edganiukov/fcm)
-[![Go Report Card](https://goreportcard.com/badge/github.com/edganiukov/fcm)](https://goreportcard.com/report/github.com/edganiukov/fcm)
+[![GoDoc](https://godoc.org/github.com/appleboy/go-fcm?status.svg)](https://godoc.org/github.com/edganiukov/fcm)
+[![Build Status](https://travis-ci.org/appleboy/go-fcm.svg?branch=master)](https://travis-ci.org/edganiukov/fcm)
+[![Go Report Card](https://goreportcard.com/badge/github.com/appleboy/go-fcm)](https://goreportcard.com/report/github.com/edganiukov/fcm)
 
 This project was forked from [github.com/edganiukov/fcmfcm](https://github.com/edganiukov/fcm).
 
@@ -10,6 +10,12 @@ Golang client library for Firebase Cloud Messaging. Implemented only [HTTP clien
 
 More information on [Firebase Cloud Messaging](https://firebase.google.com/docs/cloud-messaging/)
 
+## Feature
+
+* [x] Send messages to a topic
+* [x] Send messages to a device list
+* [x] Supports condition attribute (fcm only)
+
 ## Getting Started
 
 To install fcm, use `go get`:
@@ -34,31 +40,32 @@ Here is a simple example illustrating how to use FCM library:
 package main
 
 import (
-	"github.com/edganiukov/fcm"
+	"log"
+
+	"github.com/appleboy/go-fcm"
 )
 
 func main() {
 	// Create the message to be sent.
 	msg := &fcm.Message{
-     		Token: "sample_device_token",
-      		Data: map[string]interface{}{
-         		"foo": "bar",
-      		}
-  	}
+		To: "sample_device_token",
+		Data: map[string]interface{}{
+			"foo": "bar",
+		},
+	}
 
 	// Create a FCM client to send the message.
-	client := fcm.NewClient("sample_api_key")
+	client, err := fcm.NewClient("sample_api_key")
+	if err != nil {
+		log.Fatalln(err)
+	}
 
 	// Send the message and receive the response without retries.
 	response, err := client.Send(msg)
 	if err != nil {
-		/* ... */
+		log.Fatalln(err)
 	}
-	/* ... */
+
+	log.Printf("%#v\n", response)
 }
 ```
-
-
-#### TODO:
----------
-- [ ] Retry only failed messages while multicast messaging.