Update interpreter.py

Added the ASTProgram and ASTStmt classes to handle entire blocks of code for the interpreter.  To use these classes, the visitProgram() and visitStmt() methods were added to the Visitor classes, and the functionality of the interpreter() function (which previously handled statements) was moved to the visitStmt() method.  

This brings the interpreter structure closer to the Visitor pattern. Additionally, each statement has a semi-colon at the end (similar to SQL and many other programming languages) to distinguish between them.