Normalize `s3:` URL paths

[hannes-ucsc]

... such that we can use Unix find to mirror entire directory trees.

Use case

find . -name '*.py' -exec s3am upload --verbose --force '{}' s3://bucket/dir/'{}' \;

This currently runs

s3am upload --verbose --force ./foo/bar.py s3://bucket/dir/./foo/bar.py

which creates the . "directory" in S3. Instead it should run

s3am upload --verbose --force ./foo/bar.py s3://bucket/dir/foo/bar.py

[hannes-ucsc]

Too complicated. Punting for 1.0