Fix folder searching for versioned URLs

This fixes a bug that caused versioned URL strings to be used when searching for $folder$ placeholder objects.
Only look up credentials when necessary
Fix test_cat_version for S3
Make rm tests resilient to eventual listing consistency.
Initial tab completion support.
Currently limited to 'ls' command.
Fix tab completion argcomplete-related breaks in standalone installs
Improve ACL tests resiliency to eventual listing consistency
Print benign retry messages at loglevel debug only
Increase test_du reslience to eventual listing consistency
Fix tab completion tests to work with S3.
Added explicit warning about using streaming uploads/downloads for large amounts of data to cp help
Changed signurl command check readability function to use storage_api.httpwrapper.MakeRequest which retries on 5xx errors
Merge pull request #230 from fishjord/sign_readability_retry

Changed signurl command check readability function to retry requests
Limit number of tab complete results for performance reasons.
Better handling of some edge cases when completing Cloud URLs.

gsutil ls <tab> should do a bucket listing
gsutil ls <non URL string><tab> should do nothing as it can't match anything
Fix typo in rb command documentation.
Enable tab completion for more commands.
Increase buckets and object pagination to maximum to speed up tab completion
Add optional logging for tab completion
Remove double-gzipped files correctly when hash validation fails