begin promise refactor for File
completed File promise refactor
begin Target refactor
refactor builder & target
replace Promises with simple async callbacks
completed async refactor of build
fix watch tasks
update docs
fix typos
update date
update docs
add es6 support
update docs
Merge branch 'async'

* async:
  update docs
  add es6 support
  update docs
  update date
  fix typos
  update docs
  fix watch tasks
  completed async refactor of build
  replace Promises with simple async callbacks
  refactor builder & target
  begin Target refactor
  completed File promise refactor
  begin promise refactor for File