Add glwtGetTime

glwtGetTime returns a double precision floating point value representing
seconds since an unspecified starting point.
test/events: added time to event prints
Updated events test to work with multiple windows.

Press return to create a new window. (up to MAX_WINDOWS windows)
OSX: Allow creating 3.3+ core profile contexts

Asking for a 3.2 or newer core profile context will return whatever the default core profile context is for this system.
Asking for 2.1 or older will return the default legacy context.
Robustness and debug hints are ignored.
OS X: Added note about origin of the keycode LUT
CMake: make GLWT_LIBRARIES a cache variable

This makes it easier to include GLWT in other projects by making the
variable essentially global.