953665 - added the ability for copy operations to not also copy child units, such as a group copying its RPMs. Also restricted the fetching of existing units to their unit key fields, which reduced RAM use tremendously. Copying a RHEL6 repo went from using about 4.3GB of RAM to < 100MB.
adding unit tests
added tests and documentation
Merge branch 'mhrivnak-953665'

Leaving out copy command additions for now since 2.2 has all-new copy commands, and the new 2.2 importer may alleviate the need for the 2.1 change.

Conflicts:
	pulp_rpm/plugins/importers/yum_importer/importer.py
	pulp_rpm/src/pulp_rpm/extension/admin/copy.py
	pulp_rpm/test/unit/test_extensions_copy_units.py
Merge remote-tracking branch 'origin/master' into mhrivnak-newimporter
mongo queries for existing units are now broken into segments of reasonable size so as not to exceed mongo's allowable query size.