Not enough image on Mobile when background has Parallax

[thejimbirch]

Black background at the bottom.

[thejimbirch]

1.) Add Offset field using @albertski's Javascript. Instead of allowing entered value, add levels like we did for Speed and Overlay.
2.) Bypass the Parallax on Mobile. Add instructions in the UI, and add this to the Javascript?

var ismobile = /Android|iPhone|iPad|iPod|BlackBerry|IEMobile|Opera Mini/i.test(navigator.userAgent)
if (ismobile){
 // bypass parallax effect
}

Update the client site first, and then we can review and update this module and https://github.com/xenomedia/bootstrap_paragraphs_features