v2.0 features

This PR includes many changes:
* renameing the bundle
* move twig extension to a seperate bundle (and add dependency to it)
* replace the Date, Time and DateTime picker widgets
* remove the DateRangeType (no longer necessary for Adnubgeberator)
* (assetic) bundle the assets into packages
* (assetic) change the filters to `uglifycss` and `uglifyjs2`
* adapt to bootstrap 3
* update select2 library to 3.4.0

and many others.