Apply fixes to s3:// URL parsing under Python 2.6.

The urlparse module starting in Python 2.6 appears to be compliant with RFC
3986, while previous versions were not.  This causes a change in the
behaviro of parsing s3:// URLs, however, resulting in breakage with Python
2.6.

Try to fix this by adjusting our code so that it works with either the old
or the new behavior.