Increasing super_len compatibilty to include BytesIO and cStringIO objects.

Added a check for 'getvalue' attr, calling it to retrieve the length if we can.

We also try/except the fileno() call, which can throw
io.UnsupportedOperation for BytesIO because, well, they're not files.
Adding myself to AUTHORS.rst.
Merge pull request #1661 from mattspitz/super_len_compatibility

Increasing super_len compatibilty to include BytesIO and cStringIO objects.