Add DateTimeOffset Support

This should be all that is needed to resolve Issue #6 (and Issue #44 which is a subset of #6).  Utilizes the implicit conversion of `DateTime` to `DateTimeOffset` to simply have the existing code support both.

Other considerations:

  * Could rename the class(s) since 3.0 is already a breaking change.
  * Could update all the doc comments which currently read DateTime.  Was not sure what approach would have least user confusion.
  * Could add more tests or change the existing tests to use with DateTimeOffsets rather than DateTimes.  Added a few to show the premise, but is seemed redundant to duplicate all the tests when using the implicit conversion functionality present in the BCL.