Better handling for due dates for VersionOne tasks.

* Adds functionality to `parse_date` allowing one to specify that you'd
  like a parsed naive date string to be marked as being in the local
  timezone.
* Assumes your local timezone in situations in which you have not
  specified one.
* Adds functionality for specifying an explicit timezone name for
  imported VersionOne tasks.
Merge pull request #173 from coddingtonbear/add_timezone_support_to_versionone

Better handling for due dates for VersionOne tasks.