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 to subjectivity, with the pure intent of making increasing readability and making pull requests easier in future.

Personally, I found some aspects of the code misleading with so many cases of:

* unused variables,
* out dated comments,
* inconsistent use of `Buffer`, and
* unnecessary helper functions

As pointed out by @dominictarr in https://github.com/dominictarr/sha.js/issues/7, the reason for the inconsitent `Buffer`use (and probably the subsequent encoding functions) is because they were from a time where browserify wasn't as mature.

This PR isn't exhaustive, but, its a start.

Tested in Firefox 32, Chrome 37, and on Chrome Android 37.