Update more aggregations to work with TimeDelta, specifically Median

[Kirkman]

In the spirit of issue #735, I wonder if anyone would be willing to add support for TimeDelta in more aggregations, particularly agate.Mean() and agate.Median().

• The .Mean() update looks like it might be simple, just adopting similar changes as were made to .Sum().
• I think Median and others might be more involved.