Text secure notifications and fingerprint

This pull request includes 2 main features : (sorry for putting them together as one, I ran into issues with Storyboards and rebasing, as usual SBs don't go together well with version control ...)

## Notification Previews

Allows user to select a preferred style in Settings for which notifications should be displayed : (see #208)
* No Preview, No Name : 
![notification_no_preview_no_name 2x](https://cloud.githubusercontent.com/assets/5308013/5444538/bc99bf4c-84ab-11e4-9a55-ee2d84f61250.png)

* No preview, with name : 
![notification_no_preview_with_name 2x](https://cloud.githubusercontent.com/assets/5308013/5444545/cc3cf590-84ab-11e4-81c1-62b67c88bba9.png)

* Preview with name : 
![notification_with_preview 2x](https://cloud.githubusercontent.com/assets/5308013/5444549/d4847e76-84ab-11e4-80ce-2e0b5dc2ba8f.png)


## Fingerprint UI

Updates the fingerprint UI include : 
* Bigger text on the fingerprint
* Better support for smaller screens
* An info button that will display a few pointers as to how to properly use fingerprints to verify identity, and clarify the QR code presentation / scanning