Bug fix for edge case

In some situations, a postponed element would have a defined height, in
which case this.isInViewport(element) would always return false.

Add documentation for installing postpone with bower.

Feauture enhancement and bug fixes

Fix a bug that prevented new elements from being postponed after all the
previously postponed elements were removed from the queue.

Also, properly account for and deal with images with style property
display:none;.

First version of the API

This release is the first version of the postpone API, postpone-0.1.0.
Known issue: postpone does not currently identify when a postponed
element has the property display: none.