Fixed timing in general, and specifically the power saving mode.
There's now a distinction between what we expect the refresh rate to be (target elapsed time) and what we want the timestep to be (TargetTimeStep), which is a bit of dirty distinction and could be done cleaner, but this seems to work fine.
Found a way better place to put the power-saving stuff! Right before the SwapBuffers.
Merge branch 'develop' of git://github.com/renaudbedard/MonoGame into fez-sdl2