Option to disable animation when mobile

[gpeto91]

As requested in #38

Initialization no longer occurs automatically. You need to declare a variable in your js, passing the scrollme object as value and call the init () method manually. The init function now receives a "disable_on_mobile" parameter to prevent the animations when mobile. Also added function to check whether it is a mobile device or not.

eg.:

script.js

    var s = scrollme;
    $(document).on( s.init_events.join(  ' '  ), function() { s.init("disable_on_mobile") } );

The parameter must be exactly the string "disable_on_mobile" or it will throw an error. Also if you want the plugin to work normally on mobile, just call the s.init() function without the parameter.