Reset xml input before running iterparse on it

When using cimpy with non-file datastreams, I got errors because the xml streams were read multiple times (in _get_namespaces, _instanciate_classes and _set_attributes) but were only reset once, which caused the later function calls to fail.

This patch resets the input-stream before using calling iterparse on it. Alternatively the streams could be reset after they were consumed by iterparse.