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.

It should have been modified, right?  Just not deleted.  This added new functions to Ramda, so the min file would definitely need an update, either in the check-in or via Travis.