Datetimeindex hour of year
#### Feature description
Do you know if it is possible to extract hour of the year from a DateTime index object?
I think it would be convenient to include it in the _datetimelike_ops.

Expected behaviour:
Input: datetime => Output: hour_of_year
2015-01-01 01:00:00 =>    1
2015-01-01 02:00:00 =>    2
2015-01-02 01:00:00 =>   25
2015-02-15 08:00:00 => 1088
2015-12-31 22:00:00 => 8758
2015-12-31 23:00:00 => 8759