Added gnl_expression.h

json Objects and Values are not copyable.
Updated gnl_json.h, fixed bugs.
Json now only contains one class, the Value class.
Vectors and Maps no longer use pointers to values. Attempting to access
the Value object like an array or map will convert the Value into a json
array or json object.
Switch back to using non-pointers.
Some examples added to main.cpp for gnl_json.h