Windows fix re / vs os.path.sep

Even on Windows where os.path.sep is backslash, forward-slash is the S3 separator character.
provide a better exception / error reporting message, bug #158

Trying to cut down on the duplicates to the -bugs mailing list.
Reverted "/usr/bin/python" -> "/usr/bin/env python"

Partially reverted af984ba9
Merge branch 'master' of ssh://github.com/s3tools/s3cmd
Merge pull request #159 from mdomsch/bug/158

provide a better exception / error reporting message, bug #158
Merge pull request #154 from EnlytonJ/master

Fix needed on Windows where os.path.sep != s3_separator
Merge branch 'master' into ds-master