Clean up namespace pollution with common_iterators
Clean up namespace pollution from future/__init__.py
Add chr() to the list of disabled builtins

* Also clean up namespace pollution from future.disable_obsolete_builtins
Don't modify the importing of io. It seems to exist anyway on Py2.7.

* Previously, cStringIO was mapped to io. But BytesIO should exist there too.
Remove replacement bytes() function

This was a bad idea. It must be a class, not a function.
This test should pass:

    assert isinstance(b'abcd', bytes)

Unit test added
Add more builtins: ascii, chr, int, hex, oct
New and improved tests
Bump version to v0.2.1