Minor bug fixes, started on player.py

Merged branches 'applicative' and 'applicative-remodulate'. ...

The basic structure of the ATL interpreter now stands and (sort of)
works, still some bugs though. And obviously plenty of features left
to add (currently implemented: define-place, description, neighbours).

Ready to merge back into master.

Reorganized the modules, move DefineCommand into new module define.py ...

This avoid circular dependencies.

Various bugfixes on the interpreter.

Wrote most of the interpreter, need to find bugs now to get it to work.

Added the banner, wrote interactive starting mode.