More informative comment on error messages
Treat Tuple[T1,T2,T3] as a subtype of Iterable[T] if each Ti is
Fix https://github.com/JukkaL/mypy/issues/284
First pass at union types

Not implemented: constraint inference
Fix reversed "expression"/"variable" error message labels
Treat Tuple[...] as a subtype of Sequence[T] and Reversible[T] also
Fix subtyping for Union types

Also implement expandtype, and constraint inference (which does
nothing currently).
Basic test cases for Union types
isinstance with Union
more versatile isinstance checking
Support restricting types via isinstance if other branches return/break/continue
Tests for isinstance
Meet(Any, X) = X, not Any
Make meets from isinstance simpler but less precise
Different binder in different scopes
Support for reassignments in binder
Repeat while/for loop bodies till binder stops changing
Subtype classes on assignment
Member access/calls to union types, and hack to make error messages still OK
More clever joins that take into account declared type