first pass at manifest caching; inlined compression; get rid of deprecated compression module
discontinue storing the unit_path in the manifest.  It proved to be wonky and hard to manage.
Better manifest semantics.
validate units.
more manifest caching unit tests.
Merge branch 'master' into nodes-cache-manifest
Merge branch 'master' into nodes-cache-manifest
Merge branch 'master' into nodes-cache-manifest
Merge branch 'master' into nodes-cache-manifest

Conflicts:
	nodes/common/pulp_node/compression.py
Merge branch 'master' into nodes-cache-manifest

Conflicts:
	nodes/test/unit/test_plugins.py
docstring fix.
Merge pull request #551 from pulp/nodes-cache-manifest

Nodes manifest caching.