TST : know-fail shadding tests

The underlying issues is the changes to np.gradient in numpy 1.9.0.
Known-fail these tests so that travis is still useful while we decide
how to fix these tests.
Merge pull request #3642 from tacaswell/know_fail_color_tests_np19

TST : know-fail shadding tests
Remove use of PyCXX in core C++ extensions
Change how array converter functions are defined
Add .clang-format to repository
Restore parallel testing
Documentation
Remove some obsolete TODOs
Fix compilation
Remove debugging prints
Fix curve simplification
Fix segfault on Python 3
More Python 3 fixes
Windows-related fixes, thanks to @cgohlke
Fix test_hexbin_pickable test