Fix ColorPicker widget issues and close #13 for the time being.

Performance of color picker has been greatly improved.
Hue wheel now reacts to saturation and lightness changes.
Private components have been moved to implementation file.
Add some improvements to Painter class.

Drawing lines, rectangles and ellipses is now possible.
Also, Painter is now non-copyable as suggested in #7.
Update documentation for Painter, Layer and ColorPicker classes.
Merge branch 'master' of https://github.com/mkacz91/apricot