handle NaN case nicely in _is_sorted

Previously, the `>=` comparison would cause a
"RuntimeWarning: invalid value encountered in greater_equal".

Here, `np.amin` propagates NaN values appropriately
and doesn't cause the warning to throw.
adding test case for #2744
Added functionality for plotting the actual violin bodies to violinplot.
Added ksdensity (KDE) function in mlab and adapted _axes violinplot function to accept new KDE.
Added basic violinplot demo in examples
Added statistics lines to the violinplot function.
Added horizontal violin plot feature compatible with means, extremas and medians.
Fixed bug for regression test #1181 in scipy unit tests; ksdensity is now referred to as gaussian_kde and exists as a class in mlab.
Fixed list comp position bug and updated examples
Reverted 69b304c.
Re-added points parameter to violinplot.
Fixed several style issues.
Merging for pep8 compliance for mlab's GaussianKDE
Added violinplot tests.
updated version of test_axes.py with violinplot tests
Finished violin plot tests with horizontal feature
Initial tests for ksdensity
re worked the tests in test_mlab to work with the modified GaussianKDE
Fixed some style issues. Added violinplot to boilerplate.py
added comments for test cases referencing the origins.
Added silverman and scott tests.
implementations of the evaluate tests

Conflicts:
	lib/matplotlib/tests/test_mlab.py
Fixed kde for invalid bw_methods, misc test formatting

Conflicts:
	lib/matplotlib/tests/test_mlab.py