OHAI-95: support YAJL > JSON > JSON pure

Removes the direct dependency on a JSON library, allowing for user
choice.

Recommended use of YAJL, it's far quicker.

.gitignore

@@ -1,3 +1,4 @@
+*.swp
 pkg/
 tmp/
 coverage/

Rakefile

@@ -22,7 +22,6 @@ spec = Gem::Specification.new do |s|
   s.email = EMAIL
   s.homepage = HOMEPAGE
 
-  s.add_dependency "json"
   s.add_dependency "extlib"
   s.add_dependency "systemu"
   s.add_dependency "mixlib-cli"

lib/ohai/system.rb

@@ -22,7 +22,21 @@
 require 'ohai/mixin/from_file'
 require 'ohai/mixin/command'
 require 'ohai/mixin/string'
-require 'json'
+
+begin
+  require 'yajl/json_gem'
+rescue LoadError
+  begin
+    require 'json'
+  rescue LoadError
+    begin
+      require 'json/pure'
+    rescue LoadError
+      STDERR.puts "No valid JSON library detected, please install one of 'yajl-ruby', 'json' or 'json-pure'."
+      exit -2
+    end
+  end
+end
 
 module Ohai
   class System