Changed some python_interface functions

Changed some of the python_interface functions so that they don't return PyObject objects. This way, whatever files that includes python_interface.h won't have to know about PyObject's. More changes on this coming i nthe future.
Fixed a problem with src/Makefile.am that made the compilation process make use of incorrect Python flags.
Now the file with the user preferences also stores a number indicating the Python version being used.
Now the plugin appropriately displays the Python version it is making use of.