Replace GuvnorM2Repository's Aether operations with KIE's MavenRepository

See https://github.com/droolsjbpm/guvnor/pull/304

mfusco enhanced KIE's MavenRepository to handle the (Aether) operations performed by Guvnor's GuvnorM2Repository class. We should therefore be able to removed Aether from guvnor and re-use MavenRepository... There is one "gotcha" we may need to consider: Do we want guvnor to be dependent on drools? At the moment it is dependent on jBPM for Asset Management but some argue this should not be the case too (as guvnor is meant to be it's own entity re-usable by 3rd parties as the basis of a (Maven) project-centric workbench).

We should check with mproctor@redhat.com as to whether making guvnor more dependent on KIE libraries fits with his architectural vision.