Fix CursorIterator.next() for Python 3

It was calling next, but returning None
Modify ReadBuffer for Py3

On my Python 3.4.2 environment, UserStreaming did't work
because of an error of bytes-to-str conversion.
Merge pull request #513 from 7kry/master

Modify ReadBuffer for Py3
Merge pull request #511 from yprez/fix-iterators

Fix CursorIterator.next() for Python 3