Add DataNode that is not ephemeral.
Merge branch 'data_node' of github.com:wenbinf/ndserviceregistry into data_node
Initial Integration tests for the RegistrationBase object.

1. Bugfix the RegistrationBase.data() method to call self._watcher.get()
rather than calling self._watcher.data() (which isn't a real method
anyways).

2. Build initial RegistrationBase object integration tests. Test all of
the public methods and ensure their behavior works as expected.
Remove bogus log statement
Initial basic EphemeralNode integration tests
Remove unused imports
Pyflakes cleanup
Initial DataNode integration tests
Bugfix: Set node state before setting node data.

If the node was deleted in Zookeeper, we cannot upate
its data first -- we have to update its state, THEN
update its data.

This patches this behavior, updates the unit tests to
catch it, and adds a bit of logging.
Break the RegistrationBase integration tests out into more small tests.
Break the EphemeralNode integration tests into more small tests.
Break the DataNode integration tests into more small tests.
Add the Coverage module to the requirements for testing.

Pep8 cleanup.

Pyflakes cleanup
Documentation updates.
Merge pull request #33 from diranged/data_node

Issue #25: Add DataNode object type and appropriate set_data() methods.