ENH: GH11128 add weekday_name to DatetimeIndex and .dt
closes #11128 

some examples:

```
In [5]: s2 = s.to_frame('value').assign(weekday=s.index.weekday_name)

In [6]: s2
Out[6]:
            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
```

and

```
In [7]: s3 = pd.Series(pd.date_range('2015-09-01', '2015-09-10'))

In [8]: s3.dt.weekday_name
Out[8]:
0      tuesday
1    wednesday
2     thursday
3       friday
4     saturday
5       sunday
6       monday
7      tuesday
8    wednesday
9     thursday
dtype: object
```

Please review my code :smile: 
