Refactor, updated readme, colors+style to closer match Sparrow

* Changed the color preferences for gradients and borders to match Sparrow. [Here is the example I used](http://www.apps-island.de/wp-content/uploads/2012/03/Sparrow-Bedienelemente.png).
* Shadow opacity and sizes to match Sparrow.
* Shadow changed to white and radius 0 when active to give a bezel appearance.
* Refactor toggle code to be more DRY
* Static init method so it can't be overridden
* Added <code>-init</code> and <code>-initWithCoder:</code> for more options

Had fun with this! I almost opened Pandora's box to do some drawing for a gradient border. They need to update the API to make this easier instead of having to do EO clipping w/ 2 paths. Didn't feel like going that far this morning.