Add functionality to add grouping separators to the counts of each segment.

Added a property in the header file that turns the functionality on or off for all the segments. I chose this paradigm (as opposed to determining the functionality on a per-segment basis) for two reasons:
1. It is unlikely that one would want only some segments to show the grouping separators as this appears inconsistent to the user and is bad UX.
2. The implementation is more concise.

To achieve the desired functionality, I changed the following methods:
-countForSegmentAtIndex:
-setCount:forSegmentAtIndex:

...and added the following private method:
-setShowsGroupingSeparators:

When the property is set to YES, setting a count of 1326 will display in the segment as 1,326 (for example).
![after](https://cloud.githubusercontent.com/assets/8139148/3882038/869c022c-2193-11e4-88a4-798b0b62abdb.png)

Default behavior of this control remains the same (does not display grouping separators).
![before](https://cloud.githubusercontent.com/assets/8139148/3882039/86c15662-2193-11e4-80f1-d7268135361d.png)

I'm fairly new to the GitHub community, so I'd appreciate any feedback on my implementation of this feature. Thanks!