Refactor EAD to HTML processing in to its own include file

[ctgraham]

Currently, the rendering of EAD as HTML is contained within the theme.inc.

The introduction of caching for the XSLT transform (#69) demands that we be able to render the EAD to HTML independent of display oriented functions, like drupal_add_js().

This also better separates the functional scope, and will make merges for fixes to #70, #68, #67, and #66 cleaner.