Add "alterHeightPosition" parameter

I have added a simple "alterHeightPosition" paramater controle with precision the default position of the background image.

Exemple without parallax(**"50%", 0.8**):
http://puu.sh/ciEgw/f585fa5b17.jpg  (white margin bellow)

Exemple with my modification and parallax(**"50%", 0.8, 100**); The third parameter add +100px to the top of the background.
http://puu.sh/ciEv9/186ea60e53.jpg

We can use this new option for controle the position with precision and adjust the render :)