Add JDOM (JRuby) as XmlMini backend

I've added a JDOM backend for XmlMini which is only available when running under JRuby.

I used the form of the REXML backend and adapted the tests from nokogiri_engine_test.rb.

There is one test from nokogiri_engine_test.rb which I commented out:

on line 29 of jdom_engine_test.rb is this statement:

assert_equal 'logo.png', file.original_filename

(on line 35 in nokogiri_engine_test.rb).

I don't know where 'original_filename' is supposed to come from?

In addition there is a spurious Java stack trace in the middle of the test which doesn't cause any errors or failures.

There is a reference to this issue here:

http://jira.codehaus.org/browse/...

but that issue is closed. I'll try an get it re-opened.

Here are the results on my MacBook Pro running this benchmark:

http://github.com/stepheneb/rail...

jruby 1.3.0 (ruby 1.8.6 patchlevel 287) (2009-03-13 rev 6586)
[amd64-java], platform: Java, version 1.6.0_03-p3
----------------------------------------------------------------
  rexml      3.84
  jdom       1.25

Ruby version: MRI 1.8.6 (2008-03-03 rev 114), 
platform: universal-darwin9.0
-----------------------------------------------------------------
  rexml     13.079
  libxml     0.489
  nokogiri   7.954

which is commented out

http://jira.codehaus.org/browse/...