Project Cleanup

Chained on #8.  See diff [here](https://github.com/dcousens/sha.js/compare/dcousens:sha512...dcousens:cleanup).

Unlike #8, this is more open ended, and is mostly to make submitting pull requests easier in future.
Personally I found some aspects of the code misleading with so many cases of:

* unused variables,
* out dated comments, and
* unnecessary helper functions

As pointed out by @dominictarr in https://github.com/dominictarr/sha.js/issues/7, the reason for the encoding functions previously was from a time where browserify wasn't as mature, which is perfectly reasonable; however it doesn't mean we need to keep them?

It isn't exhaustive, but, I think its definitely beneficial for the project.