Main Window UI Improvements

This pull requests address issue #6.

I replaced the ```NSSlider``` with two ```NSPopUpButtons```, (one for hours and one for minutes).  This will update both increases usability and introduces fine grain controls over the block interval. 

The updated code continues to honor the ```MaxBlockLength``` preference (rounded down to the nearest hour).

Lastly I added the SelfControl icon to the main window for a little bit of flair.

![Screen Shot 2013-03-12 at 12 33 38 PM](https://f.cloud.github.com/assets/39421/249650/ef07feea-8b32-11e2-848f-c1ec40db118e.png)