Add support for importing Tasks from VersionOne

* Resolves #149.
Hrm -- on my first glance, I had thought that there was a big more logic in [github's get_keyring_service](https://github.com/ralphbean/bugwarrior/blob/develop/bugwarrior/services/github.py#L159-L163) method, and I had on a few occasions in the past considered refactoring the password management bits -- each service is doing ~approximately the same thing with minor adjustments.

But, as it turns out, you're right -- that method isn't doing a whole lot; I'll just follow the lead of the github service.