Make plugin AMD compatible, preserves initial background vertical position, misc. updates and optimizations

Adds AMD compatibility. (Also works fine if not using AMD). Preserves initial vertical position of the background - so you can start with a background position of '50% -100px', for instance. (y pos always assumes pixels). Updates the demo css to highlight this capability. Added a gitignore file and removed .DS_Store from repo. Optimized script by removing unneeded jquery calls and loops. Returns jQuery object to allow for chaining.