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
```

atm this doesn't work with floats (other than NaN), I have a fix makes NaN rows go to 0... this may be preferable? In fact, it would be more in line with get_dummies itself...

@jreback This could actually sneak into 0.13.1 ??
 sure....

pls dd release note / v0.13.1 (if you want) and to API docs 
 @jreback Added in release and some docs.

Also, fixed for floats/ints, so you can do:

```
In [7]: s = pd.Series(['a|b', 'a|c', 1], list('ABC'))

In [8]: s.str.get_dummies('|')
Out[8]: 
   1  a  b  c
A  0  1  1  0
B  0  1  0  1
C  1  0  0  0
```

tested for but purposely not in docs, as I don't think it's what people should be doing! ...For one thing it won't play nice with 3 (int) and 3. (float) in an object Series.
 Ahhhhhhh, I'm confused just retested the timings is now slower. Confusing (since it was just as fast when I pushed).
 meh. timings seem eratic. It's still 40ms vs 500, so it's pretty good. I just thought I had this down to 20 (for Wes' movie dataset).
 Ok, well refactored it to be nicer, and perhaps faster...

Definitely erratic though:

```
In [2]: s = pd.read_pickle('foo')

In [3]: %timeit -n 200 s.str.get_dummies('|')
200 loops, best of 3: 26.8 ms per loop

In [4]: s = pd.read_pickle('foo')

In [5]: %timeit -n 200 s.str.get_dummies('|')
200 loops, best of 3: 16.5 ms per loop

In [6]: s = pd.read_pickle('foo')

In [7]: %timeit -n 200 s.str.get_dummies('|')
200 loops, best of 3: 16.6 ms per loop
```
 and does this go in the API docs automagically? or need to define in api.rst?
 @jreback Ah, good point. I will add it to the api rst.
 Will push to master assuming the function ref is correct.
 gr8 thanks! 
 seems your tests are breaking windows: http://scatterci.github.io/ScatterCI-Pandas/
 [pushed](https://github.com/pydata/pandas/commit/f0d2c249178b9dea0b7ba049ba10af16915f4f36) (along with your fix), ooops about Windows. Sorry! Thanks for the fix!

(Edit: looks fixed in scatter)
