Prepare GLM 0.9.9.4 release

Fixed refract for negative 'k' g-truc#808

Updated readme for GLM 0.9.9.2 release

Added PDF manual

Updated 0.9.9.0 release date

Fixed glm::step that didn't work with scalars g-truc#684

Updated API doc

Removed broken code

Fixed GLM_FORCE_CXX11 with Clang and libstlc++ which wasn't using C++…

…11 STL features. g-truc#604

Updated version