Added some options

Hi Ian,

Your work on jQuery-Parallax helped me a lot. However I found that I needed more options to fit my needs, so I changed the library a bit.

Here's what I have done:
1. change initializing function to receive only one argument: option, which contains the original xpos ( I renamed offsetX), speedFactor, and outerHeight. and a added option value: offsetY, to manually offset background in Y axis.
2. Not quiet understand the use of firstTop, so I removed it.
3. I have written a new Readme.markdown , to have a complete usage guide.

Thanks for sharing :)

Best regards,
Fin