Monkeypatch patsy with with faster handling of boolean categoricals

In urbansim we know that data passing through patsy will always
be pandas Series instances. We can capitalize on that in a couple
places to avoid expensive categorical nan checking. What we do instead
is to check the dtype of the data and if it is boolean take appropriate
action without actually looking at the data.

Actually patched are the function patsy.categorical.categorical_to_int
and the method patsy.categorical.CategoricalSniffer.sniff.
Also patch categorical_to_int in patsy.build.
Merge pull request #31 from synthicity/patsy-patch

Monkeypatch patsy with with faster handling of boolean categoricals