Creates RobotoButton and adds a way to extend any TextView subclass

I created RobotoHelper, to allow the creation of any Roboto TextView subclass. I also created RobotoButton, to demonstrate how it can be done.

I didn't update the Javadocs. Sorry about that. I'm really bad with in-code documentation.

Also, if you were inspired in some way by [the code I posted on StackOverflow](http://stackoverflow.com/questions/4395309/android-want-to-set-custom-fonts-for-whole-application-not-runtime/9199258#9199258), please let me know. I think we are facing a case of circular open-source here. I'm using your project that was inspired by my own code. This is pretty awesome! Makes me feel very happy about the open source community.