ClassName helpers refactor

Use `react/lib/cx` and `react/lib/ReactTestUtils` in favour of `react/addons`, provides more efficient builds
Remove `BootstrapMixin#getClassSetFromClassName()` in favour of using `React#transferPropsTo()` to mixin `state.className`
Create `BootstrapMixin#getBsClassSet()` based on `BootstrapMixin#extendClassName()` but returns a `classSet` instead of a `className`
Refactor components to use `classSet` to define className
Minor whitespace cleanup