Replace graph with UICollectionViewController

Closes #22 

Replaced the single UIView with drawRect for the graph with a UICollectionViewController.  Each bar is now VoiceOver compatible.  The Collection View allows for animation transitions, cell reuse, and is much more reusable.  The animation during rotation needs some tweaking but I think that'll happen in another PR.  I also fixed the scale of the graph - it was adding another y-axis tick when it wasn't necessary.

This PR does not change the following:
* Stats data structures
* Logic for display of colors, labels, etc
* Remote logic

Ideally the data structures will be changed to be less ambiguous - less NSDictionary and more named properties.  At that time I'll update the graph to not be tied to a specific data structure and rather a more generic data set pattern so the graph can be reused.

Before (in WPiOS):
![img_0017](https://cloud.githubusercontent.com/assets/373903/3445482/ec218918-0137-11e4-8fcd-86fce88bd63f.PNG)

After (in Stats demo app):
![img_0018](https://cloud.githubusercontent.com/assets/373903/3445483/ec246aa2-0137-11e4-9421-a932fc5e00e7.PNG)

Does a const make sense here?