Adds a 'max' option so one can limit scrolling to an arbitrary position.

[jakubpawlowicz]

Here's an example:

var node1 = $(…)
var node2 = $(…)
node2.jScroll({
  max: function() {
    return node1.offset().top;
  }
});

Which won't allow scrolling past node1.

[jakubpawlowicz]

It's been so long for this PR so let's close it.