merge search results between local and index route

Correcting for dmp42 comments on irc
Extensions via entry points

- auto-loading of modules in the 'docker_registry.extensions' entry
  point group
- preliminary documentation
- define and encourage API on top of signals and config
Merge pull request #589 from ncdc/entrypoints

Extensions via entry points
Check listing of directory with subdirectories.

It makes appear problems with driver that return all the files matching
path, regardless of how deep they are in their filesystem tree.
Fix list_directory in dumb driver.
Merge pull request #588 from visualphoenix/feature/mirror_search

FEATURE: merge search results between local and index route
Ceph Rados Gateway and other S3 compatible APIs
Skip test_list_directory_with_subdir in S3 driver.

The base use case for test_list_directory_with_subdir is to test that
the local database is recreated correctly.  I tested this with the
current codebase and it is indeed working.

The reason this fails is that the lookup/list mocked functions are not
close enough from reality.  For more details see GH #596.
Merge pull request #596 from enovance/test/list_directory_with_subdir

Test/list directory with subdir
Add read retries to avoid problems related to s3 eventual consistency

Conflicts:
	tests/test_s3.py
Fix code formatting and removes useless imports
Fixed style violations
Add ceph-s3 config section; clean up Ceph documentation
Merge pull request #599 from carmstrong/squashed-pr-515

Ceph Rados Gateway and other S3 compatible APIs
Merge pull request #600 from johanneswuerbach/retry-s3

Add read retries to avoid problems related to s3 eventual consistency