Allow TimeDelta to have (well-defined) scales other than TAI
Ensure UTC times can be used in arithemetic with Quantity; add UT1 for TimeDelta
Change dict comprehension for python 2.6
Add test cases for Time +/- Time(Delta)
Allow TimeDelta scale=None, rewrite __add__ and __sub__ acc., more tests
Further test cases and corrections, timedelta scale=None now OK
Split __add__, __sub__ between Time and TimeDelta
Add documentation