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  0  0  0
```

*iterates over each tag filling in with str.contains.*

Edit: This works pretty fast with "few" tags (since I'm iterating over each tag), which I think is the main usecase, but I'm sure some more perf can be eeked out:
```
In [6]: %timeit genres.str.get_dummies('|')
10 loops, best of 3: 20.9 ms per loop

In [7]: %timeit genres.str.split('|').apply(lambda x: pd.Series(1., x)).fillna(0)
1 loops, best of 3: 623 ms per loop
```