Added Glfw window context creation with fallbacks.
Made the default one try OpenGl 3.2 core and 2.1, in that order.
moved info log message
create_window_default -> create_default_window
Simplified glfw_platform reexport
Made gfx::glfw::create_window take arrays of window hints, instead of
callbacks
init() is a more fitting name now
Merge pull request #87 from Kimundi/context_version_select

Added Glfw window context creation with fallbacks.