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.
Honestly, I'd rather we didn't add things to `React.*` at this point as the functionality provided here is distinct from the rest of React and could lead to confusion/problems down the line (it's unlikely in this case but there are other properties we may want to add to React without potential issues).