ENHANCEMENT: allow overridable JS/CSS minification

This allows someone to extends Requirements_Backend and plug in their own minification
of files, including CSS minification.  It also allows them to override whether or not
the header comment is written for each file.
Merge pull request #2061 from jthomerson/enhancement_3.1_allow_overridable_minification

ENHANCEMENT: allow overridable JS/CSS minification