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.
So, 2 very different items here:

1. OK to the style, thats easily remedied

2. Less so... Personally, I'd rather drop the `$` altogether and handle selection and ready/load/unload natively (even back to IE... what ... 9?). But, the dollar was already here.