Moved unresolved variable check out of Constraint.__init__ and into ExprEvaluator.__init__.
Moved error generation to separate method.
Made expr a keyword argument. Default to using self.text
Made class method to create ValueError.
_invalid_expression_error expects expr and msg to be strings
Added documentation and fixed typo.
Made error message more clear for constraints and objectives.
Ignoring pyd files (python DLLs).
Fixing generation of error for Objective
Fixed test_hasobjective
Fixed errors in Constraint. Fixed test_hasconstraint.