ClassName helpers refactor [RFC]

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 the proposed changes:

1. Use `react/lib/cx` 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`.

If you are happy with this approach, i can roll out the changes to the rest of the code and update the tests.