make the `isGenerator()` function more generic

For some background, see this Twitter conversation:
https://twitter.com/TooTallNate/status/397185683162288128

tl;dr this works with both regenerator as well as native V8 generators,
and it is probably faster in general, so hey why not. Yay JS………
fix typo in jsdoc comments
Merge pull request #39 from TooTallNate/update/is-generator-generic

make the `isGenerator()` function more generic