ENH: add weekday_name to DatetimeIndex and .dt
These names exist already in `pd.tslib` (as the below weedays calc), should just create them as a variable so can import it directly here.

```
In [1]: import calendar
In [25]: s = pd.Series(list(range(10)), pd.date_range('2015-09-01', '2015-09-10'))

In [26]: s2 = s.to_frame('value').assign(weekday=Series(s.index.weekday).map(Series(weekdays)).values)

In [27]: weekdays = [calendar.day_name[i].lower() for i in range(7)]

In [28]: s2 = s.to_frame('value').assign(weekday=Series(s.index.weekday).map(Series(weekdays)).values)

In [29]: s2
Out[29]: 
            value    weekday
2015-09-01      0    tuesday
2015-09-02      1  wednesday
2015-09-03      2   thursday
2015-09-04      3     friday
2015-09-05      4   saturday
2015-09-06      5     sunday
2015-09-07      6     monday
2015-09-08      7    tuesday
2015-09-09      8  wednesday
2015-09-10      9   thursday
```
