Implement well-small & well-large classes on well-text-elements

This is for issue #25.

It implements the ability to choose between large, normal and small Well-Text boxes. See the [Bootstrap Documentation](http://twitter.github.com/bootstrap/components.html#misc) for how it looks like.

I wanted to implement it as a radio button group, but currently there seems to be no way of doing this, so I just used a selection box.