Add support for enhanced imagery within bands of content

This adds a new plugin  called `band_imagery` which enables flipbook and locked imagery while scrolling through content.

Live Example:
http://ucommbieber.unl.edu/workspace/wdntemplates/sandbox/band-imagery.shtml

WDN classes added:
* `wdn-flipbook` Creates a flipbook of the child figure elements
* `wdn-locked` Locks a section of content within the viewport while the band is visible
* `wdn-scroll` Content to scroll adjacent to the locked content

Attributes added:
* `data-lerp-start` The selector of an element to start linear interpolation at
* `data-lerp-end` The selector of an element to end linear interpolation at

*Feedback & Review Requested On*
* class names — are the classes appropriate & do they make sense.
* appropriate markup use?

Features I'd like to add in the future:
* windowed fixed content, e.g. a scrolling window which reveals a background image 
* linear offset multiplier (increase background image offset while scrolling past content)

Inspiration & related concepts:
* Flip-book concept: http://en.wikipedia.org/wiki/Flip_book
* Linear Interpolation (lerp): http://en.wikipedia.org/wiki/Linear_interpolation
* Example in the wild: http://pitchfork.com/features/cover-story/reader/bat-for-lashes/
* Windowed fixed content: http://www.thedailybeast.com/newsweek/2013/09/13/james-p-rubin-with-the-syria-crisis-america-s-role-in-the-world-is-steadily-shrinking.html