Correct bug where `start` did not bind events

Optimize 'getElements' method

Merge @pabloz-globant's enhancement.

Account for re-rendering.

Add support for Object

Enhance postpone with UMD pattern

Document enhanced and

Fix display style computation.

This commit fixes a bug where the computed display property of a media
resource was not being properly determined. In particular, the isVisible
method was only considering the element style attribute of the supplied
element and its ancestors.

Fix event target for Firefox.

This commit fixes a crucial bug in Firefox that was preventing the
proper loading of images.

Add capability to set threshold.

This commit adds new functionality to postpone. You can now set the
threshold, the distance before an edge at which an element should be
considered to be at the edge and loaded. The threshold can be specified
in either `vh` or `px` units.