Refactor design to allow choosing between Crockford and RFC4648 base32.

I generalized the library to be able to handle Crockford base32, RFC4648 base32, and RFC4648 base32 without padding. In the process, I renamed the library from crockford-base32 to just plain base32 (since it covers both, now).

I hope you don't mind this rather large refactoring of your library (it also breaks your old API). Please feel free to just reject this pull request if you feel like it trespasses against the spirit of your library. I just thought it would be better to have this generalized version on your repo, since you're the originator.