Sandbox jruby: try #2

OK. This is a more minimal commit, not the least of which because I turned off the "strip whitespace" function from intellij :-)

It does the following things:

* Gemfile:  Don't use ruby-debug, because it keeps warning me that it doesn't work under 1.9 (??? Really ??)
* gemspec: only add marc-marc4j dependency under JRuby
* settings: Change defaults under non-JRuby to MarcReader and NullWriter and `process_thread_pool = 0`
* indexer.rb:
  * Guard against `requir`ing JRuby-only files as inappropriate
  * `require` thread_pool, which must have been coming in via the solrj_writer require
* Rakefile/tests: move jruby-only tests to *_test_jruby.rb files and change rakefile appropriately
* read_write_test.rb: Change as necessary to run under MRI

I think with these changes we should be good.