One-stop-shopping for stage dimensions

changing the stage dimensions in line 3720 of objects.js takes care of
everything
add sound library on git
add backgrounds library to git
add costumes library to git
Merge pull request #315 from snap-school/backgrounds-library

add backgrounds library to git
Merge pull request #317 from snap-school/costumes-library

add costumes library to git
Merge pull request #316 from snap-school/sounds-library

add sound library on git