Update .gitignore file.
Merge pull request #101 from DO-CV/maint-update-gitignore

[MAINT] Update .gitignore file.
Fix compilation problem on Mac OS X.
Replace 'class' by 'typename' in templates.

Just a subjective semantic preference.
Moved "colorspace.hpp" inclusion to "color_conversion.hpp".
Fix header inclusion in "test_pixel.cpp".
Fix color conversion functions.
WIP: Add unit testing for color conversion.

RGB <-> Grayscale
RGB <-> YUV
Fix implementation for conversion from yup to gray.
Add color conversion between RGB and RGBA.
Add color conversion between YUV and grayscale.
Code cosmetics.
Fix test names.
Fix compiler warnings.
Merge branch 'develop' of github.com:DO-CV/DO-CV into maint-simplify-implementation-of-color-module
Changed "hessian" to "Hessian" in docstrings.
Merge pull request #104 from DO-CV/fix-minor-typos

[FIX] Fix minor typos.
Move pixel class and functions to 'Core' folder.
Fix 'CMakeLists.txt'.
Rename test "Core_Pixel" to "Core_Pixel_Class".