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
chore(docs): fix typo in gruntfile comments

The correct word is useminPrepare
fix(build): add webapp upstream features and better coffee

Merge changes from generator-webapp and better support for coffee/js
tasks

BREAKING CHANGES: Coffee support in the Gruntfile is only added when the
coffee flag is set. Otherwise, JS is used
Fixes: #35, #440
Merge pull request #471 from eddiemonge/upgrunt

fix(build): add webapp upstream features and better coffee