Improve background tintColor

When we originally tried to change the control tintColor we couldn't get it to go light enough for a light background.  It also didn't let any actual tint colors shine through.

Before:
![iOS Simulator Screen shot 2013-03-16 7 44 51 PM](https://f.cloud.github.com/assets/1066132/267515/840fd2f2-8e93-11e2-9981-b72a9bcc16b0.png)

The gradient colors on the background components were too close to black - overriding any tintColor options.  This pull request moves those close to a middle grey, as well as changes the default tintColor to match the existing controls in the demo.  It also includes an option to customize the intensity of the gradient on the thumb.  In lighter scenarios, the gradient seemed too much.  

After:
![iOS Simulator Screen shot 2013-03-16 7 44 22 PM](https://f.cloud.github.com/assets/1066132/267516/87744a22-8e93-11e2-820b-baa0fb37f9e1.png)


JG
Thanks for making me look at this again. This is much more elegant than the other way!
=)