Changes required to support removing nokogiri dependency from math modules.
Rework blahtex support to no longer use Nokogiri or PStore cache.

The blahtex support code has been simplified to no longer cache mathml in an unbounded file-based cache, and it no longer writes tempfiles. The extremely simple XML parsing job has been handed back over to REXML.
Remove explicit Nokogiri dependency for itex2mml module.