Refactor of `Button` component

Removed `loading` and `loadingChildren` prop, this can be achevied more
React idiomatically via change the `Button`'s props via a state change.
Removed "is" prefix from `isActive` and `isDisabled` props, this more
closely resembles the DOM API.
Added `block` component to set buttons to block level/ full width.
Don't manually set properties that will be passed via
`this.transferPropsTo`.
Clean up tests, remove duplicate tests already tested else where.
Added docs for `Button` component

Documented possible states of this the button, uses the
http://getbootstrap.com/css/#buttons content as a basis for this.
Updated duplicated comment
Merge pull request #33 from pieterv/button-docs

Button component refactor + docs