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 -> eventKey
MenuItem: key -> eventKey
NavItem: key -> eventKey
TabPane: key -> eventKey
ListGroupItem: key -> eventKey

Comments or help finishing this off is welcome.
@stevoland you could avoid the whole thing if you are comfortable with another [cloneWithProps hack](https://github.com/jquense/react-clonewithprops/blob/master/index.js#L28). Save a bunch of bytes and won't have to worry about updating more and more internal react components. This approach has worked well for me thus far.