Handle while statements.  Re-order parser functions.
Handle if statements.
Remove ReadStmtNode class, reword WriteStmtNode.
Sacrifice first-born to the gods of pep8.
Add datatypes tuple to TokenType.
Allow for all kinds of expression statements.

Not just the ones that start with ID's.
Parse expressions.
Use keyword arguments in parse tree constructors.
Add cur_token() wrapper to Parser class.
Create a class method for redundant consume code.
Don't override cur_token name in Parser.expect().
Remove outdated grammar comment.
E and T rules are now (correctly) left-associative
Use `is` for comparing constant (int) values.
Change vague `ParseTreeNode.MATH_EXP` to `ARITH_EXP`.
Don't use tokens to represent variable names in the Parse tree.
Remove unnecessary local vars from Parser.factor().
Add docstrings.
Print operators more clearly.
Support all kinds of variable declarations.