Port from json to multi_json

Hello!  This patch ports rdoc from the json gem to multi_json in order to allow the use of alternative JSON parser/generators such as YAJL or Oj.

The reason for writing this patch is that recently I've been experimenting with running Rails under MacRuby.  This is currently impossible because railties depends on rdoc, and rdoc depends on the json gem.  Loading the json gem makes MacRuby blow up because its implementation is incompatible, and the authors of MacRuby have written an alternative implementation based on Apple's NSJSONSerialization to work around it.  While their implementation works fine, MacRuby users can't load the json gem.

If rdoc accepts this patch, then hopefully this will help packages that depend on rdoc, such as railties, become compatible with MacRuby.