Exposed StepMania::SaveScreenshot to Lua.  Added prefix and suffix args for setting the screenshot name and removed the unused index arg.  Added GetHighScore to PlayerStageStats and AddScreenshot to Profile so that Lua can imitate ScreenEvaluation's code for adding a screenshot to the profile.  Removed hungarian notation from variable names in StepMania::SaveScreenshot.
Merge pull request #197 from kyzentun/SaveScreenshot

SaveScreenshot for Lua