Add avatar support.

Add avatar support.
Fixes second wear page to start scroll at bottom, like Hangouts.
Cleanup notification builder, extend once.

![image](https://cloud.githubusercontent.com/assets/4513759/4435406/080e30e4-4743-11e4-93aa-d8a9395622f5.png)
![image](https://cloud.githubusercontent.com/assets/4513759/4435409/26f1b7ce-4743-11e4-9d68-9fd5444a864b.png)

Not sure about the avatar scaling on the notification bar yet. I retrieve the 400x400 avatar, as recommended by Google for wear backgrounds, but it doesn't look like it scales down for the largeIcon as I figured it would. Will look into that next.

Also, using in memory cache for the avatar, to avoid adding the extra permission for WRITE_EXTERNAL_STORAGE.
I think this is causing some problem. See for instance:
![2014-09-29 10 18 42](https://cloud.githubusercontent.com/assets/536844/4440210/248ea620-47c2-11e4-8cb1-b397765ff2b8.png)

(first notification is from the official flowdock app)