Changed POST API content-type to application/json

Fixed AddMember response parsing
added bot post message example

examples/post_bot_message/main.go

@@ -0,0 +1,20 @@
+package main
+
+import (
+	"fmt"
+
+	"github.com/densestvoid/groupme"
+)
+
+// This is not a real Bot ID. Please find yours by logging
+// into the GroupMe development website: https://dev.groupme.com/bots
+const botID = "0123456789ABCDEF"
+
+// A short program that gets the gets the first 5 groups
+// the user is part of, and then the first 10 messages of
+// the first group in that list
+func main() {
+	// Create a new client with your auth token
+	client := groupme.NewClient("")
+	fmt.Println(client.PostBotMessage(botID, "Your message here!", nil))
+}