Delete files that became directories

If /foo is created, uploaded to S3, then deleted and replaced by a
directory containing /foo/index.html, /foo should be deleted on S3.
Merge pull request #37 from willglynn/delete_files_that_became_directories

Delete files that became directories
Avoid HEAD requests

The output from the directory list is sufficient for determining a
course of action, since it includes all keys and ETags for every
object in question. Eliminating the HEAD-per-object request reduces
the total number of requests by several orders of magnitude.
content_md5 should hash the object, not the source file

In the case of gzipped content, this means that#content_md5 returns
the MD5 of the gzipped verison instead of the original. This way,
Resource#content_md5 returns the MD5 of the body S3 stores and
serves, allowing both Content-MD5 verification and ETag: comparisons
when synchronizing.