Moved GAMS execution to utils

Improved error handling and set default gams_exec to be the command "gams".
Removed some default options
Use errno module to identify WindowsError
Added resource limit again since default is too low.
Passing on keyword arguments to superclasses for all types.
Improved error handling in GdxReader and fixed bug

GdxReader now detects empty or missing symbols and raises an appropriate custom exception. Default settings for opt file have been fixed so that reslim is increased.
Added possibility to have both options and model options

Now possible to have both options of the type "OPTION x = y;" and "modelname.x = y;". Finally fixed reslim to be in correct place (can also be set in opt file, but this seems to be preferred).
Bugfix in tmp-dir and improved docstrings

Also added license text to utils.
Corrected link in README.md