Add some implementations:

- Added `Sub<Duration, Timespec>` for `Timespec`, now allowing for Durations to be subtracted (they could only be added before).  Closes #32.

- Added `Add` and `Sub<Duration, Tm>` for `Tm` allowing Durations to be both added and subtracted in a similar way to `Timespec`.
Since there does not appear to be any function analogous to `at` and `at_utc` which allows for an arbitrary timezone to be set, the adding and subtracting a duration from a `Tm` object will always result in `Tm` at UTC.   Closes #32.

- Added `PartialOrd` and `Ord` for `Tm`.  Closes #31.