Sparse get dummies perf
Previously, we did a scalar `elem == -1` for every element in the ndarray.

This replaces that check with a vectorized `array == -1`.

Running the ASV now. In the meantime, here's a simple timeit on the same problem

```python
# HEAD
In [3]: %timeit pd.get_dummies(s, sparse=True)
561 ms ± 4.96 ms per loop (mean ± std. dev. of 7 runs, 1 loop each)

# Master
In [3]: %timeit pd.get_dummies(s, sparse=True)
        2.18 s ± 273 ms per loop (mean ± std. dev. of 7 runs, 1 loop each)
```
Hello @TomAugspurger! Thanks for updating the PR.





Cheers ! There are no PEP8 issues in this Pull Request. :beers: 

##### Comment last updated on July 20, 2018 at 15:48 Hours UTC Here's the ASV (only a 3x speedup).

```
[100.00%] ··· Running reshape.GetDummies.time_get_dummies_1d_sparse                                                                                    1.79s       before           after         ratio
     [272bbdc7]       [bc658b03]
-           1.79s       1.05±0.02s     0.59  reshape.GetDummies.time_get_dummies_1d_sparse

SOME BENCHMARKS HAVE CHANGED SIGNIFICANTLY.
``` lgtm. thanks!