Fix #28: Add support for b3aggrcon and b3aggr files

With this commit it gets possible to includes b3aggr files
directly in a target platform definition.
This allow to express things like :
"I rely on what the amalgam project contributes to Luna"
by using :

include
"http://git.eclipse.org/c/simrel/org.eclipse.simrel.build.git/plain/amalgam.b3aggrcon"

or :
"I want the cdo version contributed for Kepler SR1"
with :
include
"http://git.eclipse.org/c/simrel/org.eclipse.simrel.build.git/plain/emf-cdo.b3aggrcon?id=KeplerSR1"

The b3aggrcon resource content is parsed on the fly
to produces instances of TargetPlatformDSL.

Known limitations :
- only the "repositories" and their children are used
( no platform filters..)
- only features and bundles are used
(no category mapping or exclusion list.)
- references within a b3aggr file are not resolved.

https://git.eclipse.org/c/simrel/org.eclipse.simrel.build.git/plain/simrel.b3aggr

Gives you nothing as all the actual contributions are in the included
*b3aggrcon files.
I'm not really sure this is still necessary with the xtext resource set implementation you provided.