Expose the React mounting and un-mounting mechanisms

Re: #116 

This is my initial commit, all within a `react_ujs_p.js` file so that we may discuss it further. I'll get to the tests tomorrow, though I expect the existing to pass. I do need to write a few new ones.

Currently I am using this modded version is a non-turbolinks SPA and it works fine.
Tell me if this is sacrilege: It's probably fine the way it is, at least for now. In a given app, it's going to be consistent. If you're using jQuery, it's gonna be a selection; if you're not, it's a NodeList. Isn't that right?

Why not leave it this way for now and fix it if/when it actually becomes an issue? 

If you _want_ to sort it out now then go for it, but it doesn't strike me as a 1.0 blocker since it would be _really_ hard to notice.