use peerDependencies in package.json

hi!

[peerDependencies](http://blog.nodejs.org/2013/02/07/peer-dependencies/) are great for plugins that extend functionality of other libraries. without peerDeps, it's very likely to have multiple different `backbone` and `react` instances, anytime except for when the package using `backbone-react-component` has the *exact* same `backbone` and `react` versions, which is not so good.

cheers :)