Twit 03 view pager and fragments

Adds a view pager to allow users to swipe between the home stream and the android dev stream. Swaps out all activities for fragments.

![authenticationmenu](https://cloud.githubusercontent.com/assets/3380092/3810512/1262a1a6-1c91-11e4-9e5e-aea05e78a6a8.png)
![viewpager](https://cloud.githubusercontent.com/assets/3380092/3810513/136e39ca-1c91-11e4-9c41-2f07496a4183.png)

this entire fragment logic is duplicated in `onClearCredentials`, you could extract the method and just pass in the fragment you want to replace with!