Add day_of_week* API

[cpcloud]

closes #306

[cpcloud]

@wesm @jreback Curious about your thoughts on the API here.

I've defined

• day_of_week_index(): the index of the value's day relative to Sunday which starts at 1
• day_of_week_name(): name of the day of the week

Originally I had day_of_week(name=False) which would return a different type depending on the input, which I thought might be confusing to users. OTOH it does reduce the API surface a tiny bit.

[jreback]

maybe have day_of_week be a node which can have index() or name() in it?

[cpcloud]

Interesting. This could be modeled as a struct<index: int32, name: string> logically.

So for example a user could do the following:

dow = t.date_column.day_of_week()
result = dow.execute()  # this returns a Series of dict

index = dow.index().execute()  # series of int32
name = dow.name().execute()  # series of strings

[cpcloud]

Merging on green.