ENH: add kernels for density estimation to nonparametric module.
ENH: add kernel density estimators to nonparametric module.

Supports continuous, ordered and unordered data (can be mixed).
DOC: add examples for nonparametric KDE estimators (uni/multi-variate).
ENH: add nonparametric regression class to nonparametric module.
ENH: add a semi-linear regression class to the nonparametric module.
ENH: add a censored regression class to the nonparametric module.
ENH: add significance test for the functional form of a distribution to nonparametric.

Note: Not yet completely finished, nor does it have tests.
ENH: add a single index semiparametric model to the nonparametric module.

Note: not yet completely finished, nor does it have tests.
STY: clean up missing variables, some PEP8 stuff, typos and docstrings.
STY: remove use of slicing with :: and np.concatenate.

Replaced with : and np.column_stack for clarity.  Address review comments from
Josef on PR-408.
MAINT: some small cleanups, and moving an expensive call.

Addresses one of Josef's comments on PR-408.
ENH: speed up bandwidth estimation for nonparametric.KDE

gpke() should only accept ndarrays instead of array_like, same for kernels.
They get called so often that it doesn't make sense to call asarray in them.
On a simple test asarray got called 1e5 times for one bw estimation.

Speedup of this commit ~35% on bw estimation in KDE with cv_ls for the dataset
in the tests.
MAINT: some more simplifications in kernels for KDE to speed up code.
MAINT: simplify nonparametric.tools.gpke.

Remove vectorization over number of variables K.
This is ineffective, because N >> K, and requires way too many asarray
and reshape calls.
MAINT: further simplify nonparametric kernels.
MAINT: some more cleanup, also remove print statements.

Some test failures left, mostly due to too high test precision it seems.
ENH: nonparametric speedup, replace integer by boolean indexing.

Speeds up running nonparametric.test() by ~15%.
ENH: initial Cythonization of nonparametric kernels.
MAINT: some more cleanups and small speed improvements.
MAINT: some more cleanups.