Map 'self' to 'this' in Python user code

The latest version of Filbert no longer parses 'this' to a
ThisExpression, because it's not a Python keyword.  This fix applies the
same method as Lua to map 'self' to 'this'.

'self' will be used in place of 'this' in user code:
self.moveDown()
Merge pull request #52 from differentmatt/master

Map 'self' to 'this' in Python user code