Add support for importing Tasks from VersionOne

* Resolves #149.
It is ![kind of there](https://github.com/ralphbean/bugwarrior/blob/develop/bugwarrior/services/__init__.py#L223-L226).  Do you mean moving the implementation up there?

Another option could be to use ``abc.ABCMeta`` on the ``IssueService`` base class so you would have known you still needed to implement something earlier on.