Add cJSON_Free

We shouldn't call free from memory allocated in another
unit since it may have used another memory allocator.

Change-Id: I6da15baa2cad535ca9d7b108280e193b9cdc387c
Reviewed-on: http://review.couchbase.org/34488
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>