Alternate XML parsers support for ActiveSupport through XMLmini

The XMLmini implementation now switches between a LibXML or REXML
implementation, depending on weather the LibXML library is available.
As the implementation of the XMLmini is now separated out, it is very
easy to implement Nokogiri or JREXML. The speed increase is quite
stunning (in my tests up to 10 times as fast, probably more for larger
XML files).

It falls back to the old ruby implementation (REXML) if no other
parsers are available.
http://rails.lighthouseapp.com/projects/8994/tickets/2084-alternative-xml-parsers-support-in-activesupport-for-activeresource