Better customization through use of a specific stylesheet protocol

Hi,
I just came across the lib recently and was playing around with customizing the colors a bit more. Because I like using Cocoapods, I like having the ability to customize the message views without directly altering the original libraries.

This pull request specifies a stylesheet protocol so anyone can pass in an object that conforms to the protocol and change the attributes they want. It also makes it really easy to subclass the default TWMessageBarStyleSheet and only override some of the properties. Trying to change the background color of only one of the message types is as easy as subclassing the TWMessageBarStyleSheet and just calling super for the non-modified types.

I'd be interested to hear what you think, I threw this together pretty quick this afternoon, but it appears to work for me.

I'd be happy to make more changes too.

- JG