feat(build): compile only changed coffeescript files in watch task

Use grunt-newer in both watch:coffee watch:coffeeTest

Closes #425
feature(index.html): add support for bower installed css files during minifaction

solve missing .css files during grunt:build minification due to files being installed via bower (and not copied with grunt:copy:styles)
Merge pull request #468 from pgilad/patch-2

feature(index.html): support for bower installed css files during minification
Merge pull request #453 from jjt/gruntnewer

feat(build): compile only changed coffeescript files in watch task
docs(comments): comment config blocks in gruntfile

Help let newer people know what each config section is for
fix(build): use grunt-newer for styles and jshint

Applied @tschaub's changes to generator-webapp using grunt-newer for jshint and
style copying.

See https://github.com/yeoman/generator-webapp/pull/193