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.

After some research, I found that this bug also affects some revisions of Python 2.7 as well.

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.