Bring requirements up to date
Make rsync config not check md5 unless size matches.

Prevents unnescessary recomputation of md5 during planning.
Refactored old _md5_cmp method into new _is_unmodified method,
which takes in a key namedtuple returned by _get_remote_keys.
Costmetic (pep8)
Reorganize util tests
Move multipart constants into constants file and comment.
Cosmetic (pep8)
Make S3ResponseError contain more pertinent information.
Add exception handling to _abort_multipart_upload.