Resolve a few compiler warnings.
Merge pull request #99 from DO-CV/maint-resolve-compiler-warnings-in-core-unit-tests

[MAINT] Resolve a few compiler warnings in unit tests for the `Core` module.
Rename test file for Color testing.
Create 'Pixel' data structure which replaces 'Color'.
Move color space definitions to a separate file.
Rewrite unit tests for the 'Pixel' class.
Distinguish channel conversion and color conversion.
Add unit tests for channel conversion.
Add stub for unit-testing the color conversion.
Move pixel typedefs to a separate file.
Update 'CMakeLists.txt'.
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.