add support for JRuby

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 as a dev dependency so rake can be run via Bundler

lib/pygments/popen.rb

@@ -1,6 +1,17 @@
 # coding: utf-8
-require 'posix/spawn'
-require 'yajl'
+if RUBY_ENGINE == 'jruby'
+  require 'open3'
+  def popen4(script)
+    streams = Open3.popen3(script)
+    streams.unshift(streams.pop[:pid])
+    streams
+  end
+  require 'multi_json'
+  (Yajl = MultiJson).engine = 'json_gem'
+else
+  require 'yajl'
+  require 'posix/spawn'
+end
 require 'timeout'
 require 'logger'
 require 'time'
@@ -13,7 +24,7 @@ class MentosError < IOError
 # Python process.
 module Pygments
   module Popen
-    include POSIX::Spawn
+    include POSIX::Spawn if defined?(::POSIX::Spawn)
     extend self
 
     # Get things started by opening a pipe to mentos (the freshmaker), a

pygments.rb.gemspec

@@ -14,9 +14,14 @@ Gem::Specification.new do |s|
   s.email = ['[email protected]']
   s.license = 'MIT'
 
-  s.add_dependency 'yajl-ruby',   '~> 1.2.0'
-  s.add_dependency 'posix-spawn', '~> 0.3.6'
-  s.add_development_dependency 'rake-compiler', '~> 0.7.6'
+  if RUBY_ENGINE == 'jruby'
+    s.platform = 'java'
+    s.add_dependency 'multi_json', '~> 1.12.1'
+  else
+    s.add_dependency 'yajl-ruby',   '~> 1.2.0'
+    s.add_dependency 'posix-spawn', '~> 0.3.6'
+  end
+  s.add_development_dependency 'rake'
 
   # s.extensions = ['ext/extconf.rb']
   s.require_paths = ['lib']