Fix command argument appending function

Previously we always returned the first element. Now we return the
last one (the one we just allocated), which was intented in the first
place.
Merge pull request #297 from neithernut/fix-command-argument_appending

Fix command argument appending function
Add deserializer callback interface
Add deserializer callback source file (stubs) and CMakeLists.txt entry
Add implementation for NULL callback
Add implementation for boolean callback
Add implementation for integer callback
Add implementation for string callback
Add implementation for start_map callback
Fix: Name of a command must be assignable.
Add implementation of map_key callback
Add implementation of end_map callback
Add implementation of start_array callback
Add implementation of end_array callback
Set callback table
Add bracket count initialization in deserialize_state_new()
Add key for stack position argument type
Add state for stack position argument type state
Initial json deserializer testing setup
Add test for init/deinit deserializer