ENH get_dummies str method

fixes #3695.

```
In [8]: s = pd.Series(['a|b', 'a|c', np.nan], list('ABC'))

In [9]: s.str.get_dummies('|')
Out[9]: 
    a   b   c
A   1   1   0
B   1   0   1
C NaN NaN NaN

[3 rows x 3 columns]
```

*iterates over each tag filling in with str.contains.*