Added GLFW_DONT_CARE.

This changes the behavior of framebuffer hints set to zero.  Before,
zero meant that any value was acceptable.  This changes zero to be an
explicit preference and adds GLFW_DONT_CARE to mean that any value is
equally acceptable.

Fixes #70.
Added GLFW_DOUBLEBUFFER.
Added glfwGetKeyName.
Added glfwSetSizeLimits.
Added position testing to windows test.
Added GLFW_FLOATING.

Fixes #23.
Updated glad.
Fixed plural forms on key/button arrays.
Made windows test show all windows at once.
Formatting.
Documentation work.