ClassName helpers refactor

Hi!

I have put together a RFC for a refactor of the way classNames are used as discussed in #3. I think this way provides more flexibility for components with a minimal need for additional boilerplate code.

This are changes include:

1. Use `react/lib/cx` and `react/lib/ReactTestUtils` in favour of `react/addons`, provides more efficient builds.
1. Remove `BootstrapMixin#getClassSetFromClassName()` in favour of using `React#transferPropsTo()` to mixin `state.className`.
1. Create `BootstrapMixin#getBsClassSet()` based on `BootstrapMixin#extendClassName()` but returns a `classSet` instead of a `className` and doesn't mixin `state.className`.
1. Refactor components to use `classSet` to define component className's.
1. Minor whitespace cleanup.