Add common interface for JSON de/serializer + stubs

This "common" interface handles the yajl handle.
Add implementation of serializer_yajl_state_new()
Add public JSON deserializer interface definition + stubs
Add deserialize callback implementation
Add implementation of ws_serializer_json_deserializer_new()
Add deserializer state helper interface + stubs

And appropriate entries in CMakeLists.txt
Add implementation for deserializer_state_new()
Add implementation for deserializer_state_init_cmds()
Add implementation for deserializer_state_new_arg()
Add implementation for deserializer_state_mod_last_arg_set_direct()
Add implementation for deserializer_state_mod_last_arg_set_indirect()
Add implementation for deserializer_state_mod_last_arg_set_null()
Add implementation for deserializer_state_mod_last_arg_set_bool()
Add implementation for deserializer_state_mod_last_arg_set_int()
Add implementation for deserializer_state_mod_last_arg_set_string()
Add implementation for deserializer_state_push_cmd()
Merge pull request #254 from matthiasbeyer/add-json_deserializer_interface

Add JSON deserializer interface definition