Add simple test-cases for upnp.core.action.Argument.
Add some first test-cases for upnp.core.action.Action.

Many more are needed.
Fix log-level: It's an error if the SSDP server is already running.
Enh: Log original error message if starting the SSDP server fails.
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
Merge branch 'maintain/0.6.x' into develop

Conflicts:
	coherence/upnp/core/ssdp.py