Make all/get/themes work.  "all" would be better called either, as it
just resolves across either graph.  It doesn't combine results which
is what all would imply.

Refactored the responsibilities between ThemeRepository and
ThemeService so that the service is now responsible for trying to
convert id's into resources.

TODO - refactor DocumentService/DocumentRepository and their specs to
be similarly structured.

All specs passing.
Merge remote-tracking branch 'origin/master'