fix linking errors for Mac OS X, as far as dependencies are concerned
check for CLANG on OSX. If available, set the std++ lib so it solves the
final linking error coming from sfml actually (which uses std:
'sf::Font::loadFromFile(std::string const&)", referenced from:...')

This solves compiling and linking but generates lots of warnings
(most unused-parameter)
Minor formatting adjustment.
Added Stefan Hendriks to AUTHORS file.
Split Renderer into an interface and 2 instanciations using VBOs and vertex arrays, fixed corner case bug with Canvas, fixed viewports not propagating properly, fixed index count not being reduced in renderer when Primitives are removed.
Generalized Renderer textured quad creation from CreateImage into CreateSprite allowing Engine developers more freedom to manage resources and convey their usage.
Fixed FindSFGUI.cmake ignoring standard arguments (applied patch from #674), removed obsolete UpdateViewRect() from sfg::Desktop (#682), allow creation of RadioButtonGroups in advance (#685).
Fixed Renderer crashing when drawing without any textures loaded into the atlas by forcing creation of the pseudo texture by the Renderer during its construction.
Fixed loading fonts in renderer (#701, patch by Dematos).
Minor formatting adjustment.