Replace json decoder with json-c library

The license of the current json decoder used in the ext_json library
presents packaging issues for debian. Specifically the addition of
"The Software shall be used for Good, not Evil." to the MIT license 
by JSON_parser.cpp.  This patch adjusts hhvm to utilize json-c, an MIT 
licensed library used in the pecl jsonc extension. The integration code here
is a direct port of the PECL jsonc extension by Remi Collet.

There are two main outstanding issues:

* hhvm has its own json encoding system which is not being replaced,
  but it makes a single call to utf16_to_utf8 from the previous
  library which is still included in this patch.

* There are a few facebook specific changes to the json decoder to
  implement an FB_LOOSE flag allowing json like '{z:1}' to be valid,
  this has not been reimplemented.

There are probably a variety of smaller issues as well, the patch merely
appears to work in some circumstances and still needs careful review.

Before i finish this patch I wanted some feedback on the two points above,
most specifically about FB_LOOSE.  I know facebook has an internal hhvm
repository, would it be reasonable to strip FB_LOOSE and JSON_parser.cpp
from the open source repository, leaving it only in your internal repo?