Add DatetimeIndex.day_of_year Method

[dhirschfeld]

PeriodIndex has a day_of_year method, but DatetimeIndex doesn't. This makes it hard to write generic code which can take either.

Given they both have dayofyear attributes it may be best to remove the day_of_year method so that there is only one obvious way to do it.

[dhirschfeld]

@wesm just wondering what the appropriate forum is to bring up issues which may or may not be bugs since I'm still learning the package. Is it best to bring them up on the ML first and let you create issues if required or should I open the issues directly (like I did this one) and let you triage them if they turn out to be... non-issues?