Implemented a more flexible blending system (fixes #298)
Created branch for experimental blending mode API

See issue #298.
Based on branch 'new_blending_api' of github.com:tomgalvin594/SFML.
Fixed compile error (trailing enum comma) and other minor things
Omitted "Blend" prefix for nested BlendMode enums

BlendMode::Factor instead of BlendMode::BlendFactor
BlendMode::Equation instead of BlendMode::BlendEquation
Improved formatting
Improved documentation
Passed BlendMode objects by const-reference
Reviewed the sf::BlendMode class (added a constructor, made minor modifications in comments)
Merge branch 'blendmodes'

Introduces the sf::BlendMode class that supports separate blend functions and equations.
Replaces the old sf::BlendMode enum.

Further changes:
* Fixed merge conflicts resulting from GLExtensions in mobile branch
* GLExtensions: Added GLEXT_blend_equation_separate
* Added file BlendMode.cpp
* Replaced occurrences of sf::BlendMode value parameters with const-references
* Adapted rendering code in sf::RenderTarget