Implement UnboundLocalError

[yryuvraj]

Resolves #1

Note : There might be some faults in the code. Please review and suggest changes.

[tusharsadhwani]

• The scope checker should be called in the main function inside interpreter.py, not here.
• An ast.NodeVisitor will not work because we aren't using Python AST nodes, we have our own AST. To get a visitor, create a Visitor class (in a new visitor.py file) and move the visit() method from Interpreter into that class.
• Nodes currently don't have a line and column field.

Lot of mistakes here, I haven't properly looked at the visitor logic yet.