Ivy Buildlist Revision Support

Currently buildlist does not support revisions while filtering leafs and roots.

The leaf and root attribute should handle specifying revisions (and even organizations) in the form of (org#module#revision,org#module#revision,...) and should return a list also of the form (org#module#revision,org#module#revision,...).
This is crucial when new dependencies are introduced in new releases of a certain project.

A use case for this is in continuous integration scripts, where a change in a certain org1#module1#revision1 should trigger a recompilation of a dependent org2#module2#revision2 (All other org2#module2 projects should not need rebuilding)