React 0.12 support

Provide React 0.12 support in all react-bootstrap components.

BREAKING CHANGES:
It's no longer possible to use key indiscriminately within React components, so I had to make the following changes to component props:

Panel: key -> selectKey
MenuItem: key -> selectKey
NavItem: key -> selectKey
TabPane: key -> selectKey
ListGroupItem: key -> clickKey (named this 1 click because it's used in the onClick handler, while all the others are used in onSelect handlers)

Comments or help finishing this off is welcome.
This file can be imported from react by ```require('react/lib/joinClasses');```, so is ```classSet``` and ```cloneWithProps```.