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.

#661