Main loop frame rate is now adaptive as shown in class slides, plus a frame rate cap.  Game time factor added but because there is no actual update yet it isn't used.
Added a framerate indicator that's disabled by default.  Enabled by toggling FRAME_RATE_DISPLAY at top of helloworld.cpp.  Displayed framerate is an average calculated over the last FRAME_RATE_AVGFRAMES frames.
Merge pull request #3 from fenaren/adaptive_main_loop

Adaptive main loop with framerate indicator