Fixed typechecking on BinaryExprs

Relational operators need to return a boolType, rather than the compared types.
Moved method signature checking into MethodDecl

Where it belongs.
Added checks in typeCheck for error and class types
Disallow void-typed field declarations
Fixed visitBinaryExpr checking method

Now divides up cases by operator type, and displays more useful error messages.
Specify method in MethodDecl error msg
Added preliminary SourcePosition implementation
Implemented specific Reference subclasses

Specifically, ThisRef, ClassRef, MemberRef, LocalRef, and DeRef.
fixup! Implemented specific Reference subclasses
Visit the return expression of a MethodDecl in ID
Eclipse formatter
Removed debug printing from IdTable
Added check for assignment to a method
Don't display non-integer type error on ERROR type
Fixed infinite recursion bug in MemberDecl visitor
Fixed null type value in ClassDecl bug