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.

For `dayofweek`/`day_of_week` we also already have `weekday`, so this is adding a third alias for that one .. 
So I am a bit torn on this one (although I like the fact that we try to standardize the namings)
 @jorisvandenbossche I think we should deprecate the original ones in 0.18 anyhow (eg daysinmonth)
 Maybe we could also remove these from the dir to avoid all the duplicates in tab completion?
 there is a test in test_series for dt accessor that needs updating as well
 yep @jorisvandenbossche agreed
 I think choosing one prefered name per attribute, and remove all others from the docs (only mentioning them somewhere as being aliases) and tab completion, is maybe a less 'aggresive' way for striving for consistency instead of really deprecating them
 sure that's fine too 
 @emilydolson you will need to define

def _dir_deletions()

which returns a set of the attributes to remove
 I've got the tab completion fixed for DatetimeIndex and PeriodIndex, but defining `_dir_deletions()` in Timestamp doesn't seem to affect the return value of `__dir__()`. Is there something about Cython that might interfere with _dir_deletions? 
 so the issue here is that `Timestamp` needs to inherit from `PandasObject`, which is in `pandas.core.base`. But you can't simply include that as it would have a circular dependency.

So here is what we need to do.

Move from `pandas.core.base`, `StringMixin` and `PandasObject` to a new module, call it `pandas.core.object`. import this to `pandas.core.base` (you _may_ want to change imports of `PandasObject` elsewhere in the codebase). Then you can import this as a mix-in for `Timestamp` (in tslib.pyx).

long-story-short this will then work.
 can you rebase / update

also let's deprecate the old names (e.g. w/o the underscores).

You will need to put the warning in the accessor (and probably create a specific function/property for them in order to do this as they are defined by the `_field_accessor` ATM)
 can you update
 @emilydolson can you rebase/update according to comments
 @emilydolson can you rebase/update according to comments
 closing, but pls reopen if you'd like to update, which would be great.
