Initial drop of rewrite for recast-based transpiler

This is the initial drop of the rewritten Broccoli plugin to work with the Recast-based es6-module-transpiler.
Add code comments for rewrite
Update code comments and fix typos
Switch to using symlinks for faster builds
Update es6-module-transpiler to 0.6.0, source maps

This now outputs source maps!
Back-out use of symlinks until issues are resolved

This package is still prepped to use symlinks once all of the Broccoli + symlink issues are resolved:
https://github.com/broccolijs/broccoli/issues/179
Merge pull request #7 from ericf/recast

Rewrite for recast-based es6-module-transpiler