Fix __version__ import.

There were circular import issues most notably during install. `setup.py` now grabs the version information without having to import the module (which led to importing other modules which isn't appropriate during setup.py activities).
Tidying - reorder params and add a documentation line for third param.
Merge pull request #19 from joncotton/fix_version_import

Fix version import. `__init__.py` is the canonical location