Use `regenerator` instead of `regenerate` more consistently

This is more consistent with the name of the project and avoids ambiguity with my Regenerate package on npm (http://mths.be/regenerate).

I guess some ambiguity is there anyway, seeing as the project names are so similar and they both generate JavaScript source code, but there’s no need to make it any worse :)

So, this patch:

* Renames the binary to `regenerator`
* Changes the internal function name to `regenerator` instead of `regenerate`

I’d really appreciate it if you could merge this, bump the version number, and publish it on npm afterwards.