Simplify Gruntfile.js

Current Gruntfile.js is growing fast with every library that we add and every
bundle that we create. It almost become impossible to manage 1000 line long
file.

things in this pull request:

- created MockupGrunt helper
- rewrite Gruntfile to use MockupGrunt
- fixed upstream repos where bundled resources go
Merge pull request #273 from plone/garbas-gruntfile-rework

Simplify Gruntfile.js