Turn Documentation into standalone website, add Overview Page

This can now be built standalone and then copied into the correct docs
folder of the website SVN.

The index page now has a short overview and a table of contents.
[scala] Add and* methods to AggregateDateSet

And create AggregateDataSet in the first place, to add the methods to.
[doc] Unify "DataSet Transformations" page
[doc] Add automatic generation of Scaladoc
[scala] Add package documentation for package api.scala