Unbind events

[asafdav]

Hi, thanks for this awesome lib.
I solved a memory leak you had (you didn't unregister the scroll and touch events).

[ghost]

Thanks for the pull request! I'm getting a few javascript errors with this commit though. It looks like you may have copy pasted the bind logic from the parallax background attribute to the parallax attribute without changing out some values.

[asafdav]

Hi, thanks for the review.
Let me know if it works better now.

[ghost]

It looks like most of the correct directive values have been switched out, but I'm getting more errors while testing these changes. It looks like the parallax and parallaxBackground directives initially had a different attrs variable in the link function signature. I fixed the JS error by changing out the attrs variable with $attrs in the parallax directive, but that started throwing more errors in the unbind function.