Patch committed in r62778.

Antoine wrote:
> Also, I stand by the suggestion I made about the resizing logic, but it
> shouldn't be a showstopper either. We can improve that later.

I made your suggested change to the resizing logic. Thanks again for the
review! 

> By the way, is someone already working on a C-accelerated TextIO?

Yes. I was waiting to commit the C optimization for io.BytesIO, before
continuing the one for io.StringIO. The implementation is mostly
done---it just needs to be updated with respect to the recent changes to
TextIO.