Create features for each Scala versions

This will allow to install multiple versions of Scala on
the same Eclipse installation.
The main sdt build phase now only depends on p2 repos, external or local
created in the previous phases.
Some version definition
Tweaks the arguments in the build script
Creates a Scala 210 jars bundle

To be used instead of the normal bundles until when are able to support multiple bundles of Scala
Fixes scala211 source feature label
Adds computeSourcePath documentation