chore: prfun and jshint Git pre-commit hook

jscs is not yet added to pre-commit hook because of jscs-dev/node-jscs#862.
feat: add a new Payload class + tests

Also refactor existing code to make use of prfun.
chore: remove es6-promisify in favor of prfun
chore: refactor stuff and rewrite into bemuse-packer
feat(BemusePacker): actually write to files
fix(BemusePacker): fix logic for checking the package size
fix(BemusePacker): fix MIME types for bms, bme, bml, and pms format

Since the specification does not mention any MIME type, let's invent a
vendor MIME type: application/vnd.be-music-source.
Merge pull request #10 from bemusic/bemusepack2

Implement BEMUSEPACK 2