ENH: Standardized timeseries accessor names, #9606
In accordance with https://github.com/pydata/pandas/issues/9606, I added `day_of_week` and `day_of_year` accessors to DatetimeIndex, Timestamp, and Period. Since https://github.com/pydata/pandas/issues/9606 didn't mention `weekofyear`, and there's already `week` as a replacement for that, I didn't add a `week_of_year`, but I'm happy to.

I updated the tests and docs, but this is my first PR to pandas involving actual code, so let me know if I missed anything.
