Adaptive main loop with framerate indicator

Okay guys here's something to check out.  This branch makes the main game loop adaptive as shown in the class slides, plus a cap on the max frame rate.  Max frame rate is defined in MAX_FRAME_RATE at the top of helloworld.cpp.

It also adds a textual framerate indicator that's disabled by default.  When it's enabled (toggle FRAME_RATE_DISPLAY to do this) it shows up in the upper-left corner of the window.  The actual number is a running frame time average over the last FRAME_RATE_AVGFRAMES frames.

Let me know what we think.  If we want this I'll merge it.  Comments on the implementation are welcome also.