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.

>> I also want __ramda.min.js__ to be updated each time.

> It's not a realistic option if it will add 20 kilobytes per commit. 

Yes, but if it's Travis doing it on a successful build, I don't think that's a concern.

> Can you remind me why we include a minified version at all? 

Well, if for no other reason, then because that's what the CDNs want.  And they pull it out of our NPM repo, so it has to at least be there.  But it's just been such a long-standing tradition, that I didn't really even consider _not_ supplying one.