Make all the multibyte codec tests pass.

Changes to io.py, necessary to make this work:
- Redid io.StringIO as a TextIOWrapper on top of a BytesIO instance.
- Got rid of _MemoryIOMixin, folding it into BytesIO instead.
- The read() functions that take -1 to mean "eveything" now also take None.
- Added readline() support to BufferedIOBase. :-(