modify StretchNoLoop and StretchRewind to only run certain commands if they exist.
fixes issue #337

I also modified StretchNoLoop to start at the beginning of the movie to prevent it from freezing on the last frame (which I'm assuming is not the intended behavior; let me know if that's wrong)
update changelog with the last fix; SM5 beta 4a shortly.
changelog cleanup on aisle 5; this feature was just reverted, oops sorry
Set StepsTypeStr in Steps::CreateBlank.
Merge branch 'UnknownStyleSupport' of https://github.com/kyzentun/stepmania
Fixed UnknownStyleSupport.
Don't crash if AI.ini doesn't exist.
Added StageStats:GetStepsSeconds lua function.
Merge branch 'master' of github.com:stepmania/stepmania