"print" should use __str__ or __repr__ when available #24

The builtin print() method is currently printing values like this (see https://github.com/go-python/gpython/blob/master/builtin/builtin.go#L192):

fmt.Printf("%v", v)

The result is that some python objects are printed differently than expect:

None: print {}
True: print true
False: print false

The print method should probably use str or repr when available. As an alternative all types implementing str should also implement String()