Improved iTunes/MP4 (.m4a) tag mappings

Renamed iTunes/MP4 native tags from 'm4a' to 'iTunes MP4'; #13.
Combined unit test for MusicBrainz, to test common mapping using various input formats.

skip covers

• Added option to skip album-art (covers) #11
• Documented and fixed the native option #12
• Dropped browser distribution support

File access, promise based, Mocha

• Enable direct file access in addition to streams. Files have the benefit that it allows random file access (e.g. skip chunks of data) #7.
• Replace callbacks and events with promises.
  • Switch from strtok2 to strtok3
• Make use of unit-test framework, #8.
  • Used Mocha.
• Support M4A/iTunes tags, #13.

Fix memory leak

Ref: pull request #5

0.6.3: // Fix issue #3, caused by a wrong "this" scope. As a workarround I r…

Fix:

• uncaught exception (issue #2)
• drag/drop example (issue #3)