Added Unique Notification Icons & BigText Support

Unique Notification Icons:

Modeled off of Facebook / Google Hangouts, I added a way to have custom notification icons for each separate notification (i.e. if Jan sends a message, display Jan's picture as the notification icon, if Bill sends a message, display Bill's picture as the notification icon). You would just need to specify the url for the image as the "icon" variable passed from GCM.

Ticker:
Added the message text into the ticker so it's displayed across the top when a notification comes in.

BigText Support Added:
Added in support for BigText where users can expand the notification and see more of the message. For more information about BigText click here: http://developer.android.com/training/notify-user/expanded.html