resolves #99 add support for JRuby

[mojavelinux]

When RUBY_ENGINE == 'jruby', do the following:

• map popen4 function to Open.popen3
• map Yajl to MultiJson and set MultiJson engine to json_gem
• update gemspec to build -java gem when running under JRuby
• drop posix/spawn and yaml dependencies and add multi_json

Additionally, add rake and test-unit as development dependencies so rake and tests can be run via Bundler, respectively.

[mojavelinux]

Please note of the following facts:

• This patch does not change how pygments.rb functions under MRI in any way.
• All the tests run successfully under JRuby 9.1.2.0
• Installation and requiring of the library works exactly the same on all platforms (Ruby automatically selects the right version to install and load).
• This gem must be released using JRuby (it will automatically upload the platform-specific variant).

[mojavelinux]

I recognize that Rouge is now the preferred gem over pygments.rb. However, there are still integrations which rely on this gem that haven't had a chance to make the switch. Support for JRuby would really help in this transition period.

[gfx]

This gem depends on the very POSIX process model. If you want to execute pygments for JRuby, it should be better that you call pygmentize(1) command directly.

Or, consider https://github.com/jneen/rouge, which is written in pure Ruby.

[mojavelinux]

Seriously?!? Come on. This fix is critical for compatibility across Ruby platforms (esp during the transition to Rouge, which can't happen overnight). I spent a lot of time to figure this solution out and I think it is very reasonable. Can you please respect that effort and not close the door on JRuby? I urge you to reconsider.

[gfx]

Hmm, okay, will try to get compatibility with JRuby and keep it as much as possible.

However, dynamic dependencies in gemspec are useless. Can you fix some points?

• Make POSIX::Spawn optional and add a configuring point to select POSIX::Spawn or pure Ruby open4
• Always use MutiJson

[mojavelinux]

Thank you. I'd be glad to update the patch. Stay tuned.

[gfx]

The idea of this PR is included in #170. Thanks to the suggestion.