Added source_delay property to SourceMixin which means it'll now appear
everywhere. Default is 0.01 which is just enough to drop CPU usage while
remaining responsive enough for the majority of purposes.
This removes the circular dependency introduced in PR#137. This also
fixes up an issue in the base meta-class which meant it wasn't working
in Python 3 (only Python 2), and adds a bit to the meta-class to allow
docstrings to be inherited (taken from the rest-docs branch).
