Update to use hook_imagex_variables().
Adds mediaelement to build, dependency, and default variable.

Sets up mediaelement for standardizing video and audio tag
support across browsers (html5).
Add documentation about configuring video embeds post-install.

Includes screenshots of working configurations.
Add documentation for end results and swap other images.
Update documentation to reflect preview image instead of player.
Update docs images with annotations.
Merge pull request #15 from kevinchampion/mediaelement

Feature: Add mediaelement.js and video embed documentation.