Suggestion — R.invert and R.invert.many

I just converted a project of mine from lodash to ramda and couldn't find this function in the docs. If this is already in there, please let me know and I can close this out.

Anyhow, I figured I'd whip something up quickly and see if you guys like it. If you do, I'll write some tests to make it more official. In the mean time I scanned your code and did my best to stick to your coding style — assuming I didn't miss something similar to R.invert — let me know if you'd prefer any code changes.

@buzzdecafe and I discussed this briefly on [Gitter][1] a few days ago.

> @davidchambers:
> Once we merge #618, do we want to update __ramda.js__ with each commit or with each release? For certain use cases (e.g. jsPerf) having the latest code available at all times in a single file is great. I'm in favour of updating the file with each commit. If others are of the same mind I'll update the PR so that `npm test && make lint` will fail if __ramda.js__ is out of date.
>
> @buzzdecafe:
> each commit sounds good.

I don't feel strongly about this. If you're happy to give up the ability to point to the latest version of __ramda.js__, we can switch to updating the file with each release instead.


[1]: https://gitter.im/ramda/ramda