Reject '--' in the new PA2 grammar
Added AST classes to repo
Added SourcePosition for use with AST classes

SourcePosition implementation is the same as the one used by
TriangleTools. Actual tracking of source position in the parser is a TODO.
Migrated to new Token class implementation

Token is now an actual class instead of just an Enum. The Enum that used to be named Token is now called TokenType and is defined within the Token class. In addition, Token instances now track token spelling in addition to type.
Added AST construction to parseX() methods

Parser.parse() now returns an AST (specifically, a Package instance at runtime).
Proper operator precedence has not yet been implemented yet -- parsing is left-to-right.
Fixed Token spelling bug with subtraction operator
Cleaned up old TODOs