Add support for importing Tasks from VersionOne

* Resolves #149.
I think there's a problem here.. I have to change the way that these 'service URLs' are generated in order to get ``bugwarrior-vault`` to work.   See how the ![github service does it](https://github.com/ralphbean/bugwarrior/blob/develop/bugwarrior/services/github.py#L159-L163) for an example.