support listing buckets by delimiter, with tests

This commit extends the GET method on buckets so that objects
can be listed using a delimiter, such that the result contains
not just a list of keys, but also (possibly) a list of common
prefixes.

For reference, see http://docs.aws.amazon.com/AmazonS3/latest/API/RESTBucketGET.html
fix (Python!) typo in sorted_object_list.rb
add delete objects
Merge pull request #1 from pola88/add_delete_objects

add delete objects
Correctly handle 100-continue header.
DeleteObjects should return a 200 response.
Removed .rvmrc
Added a test for delete multiple objects.
Merge branch 'delete-objects'
Merge branch 'list-by-delimiter'