API: datetimeindex subtraction
from [SO](http://stackoverflow.com/questions/27502406/convert-pandas-datetimeindex-to-fractional-day-of-year) and previous discussions

currently you cannot subtract 2 datetimeindexes ('-' is actually a set op). This is the ONLY valid numerical type of operation (e.g. you cannot add dti).

We current support pretty much the fully gamut of obvious ops, with `dti-dti` being the exception.

full list of tests are here: https://github.com/pydata/pandas/blob/master/pandas/tseries/tests/test_base.py#L434

I will propose

`dti.sub(dti)` to rectify this

thoughts
