Add isActive, isLoading and loadingChildren props to Button
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` and doesn't mixin state.className.
Refactor components to use `classSet` to define className.
Minor whitespace cleanup.