Parallax Background

Added a three layer (only two are fully implemented at this point)
background with a parallax effect.
Implemented third layer

Third layer implemented, but image is not set and it is not added to the
level.  If we need it we can use it when designing levels.
Added Image and Y-Position Parameters

Added image and y-pos parameters so the three bg layers can be used for
multiple backgrounds
Merge pull request #11 from OnStudios/parallax

Parallax