Add doc-strings.
Add test-cases for upnp.core.action.Action (without SOAP).
Fix a test-case in test_didl.

This test-case expects a call to raise an AttributeError, so use
assertRaises() instead of self-implemented code.
Merge branch 'feature/test_action' into maintain/0.6.x

Conflicts:
	coherence/upnp/core/test/test_action.py