Fix multisegment imports in some Python 2 revisions.

I discovered upon deploying waitress 0.8.4 in production that there was
an issue with waitress.runner whereby __import__ would raise the
following error:

    TypeError: Item in ``fromlist'' not a string

The error didn't manifest itself under Python 2.6.8 (which is what the
test suite was ran against), but did manifest itself under 2.6.6 (the
version I was using in production). I believe this is a consequence of
the use of the unicode_literals feature import, which is required so
that it can run on both Python 2 and Python 3.

While I was able to reproduce the error reliably, I wasn't able to track
down of any mention of this issue in the NEWS file. I think it was
silently fixed some time between the release of 2.6.6 and 2.6.8.

Given this is something of a weird, obscure bug, I've included
documentation of the issue in the resolve() function as the need for the
cast is non-obvious.
Merge branch 'unicode-imports' of github.com:kgaughan/waitress into kgaughan-unicode-imports
add changelog entry