Fix loose deps

- hard pin dynamically inserted libraries
- fix deps loose mechanism so that:
 * short versioned libraries are scopped as the rest
 * additional restrictions (blacklists for eg) are ported over
Merge pull request #587 from docker/fix-deps

Fix loose deps
README.md: Fix nginx conf URLs
Merge pull request #591 from franklinhu/fix-nginx-conf-url

README.md: Fix nginx conf URLs
Explicitly require setuptools

Add explicit dependency on setuptools, since we use pkg_resources from
it.
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 #592 from ncdc/require-setuptools

Explicitly require setuptools
Merge pull request #589 from ncdc/entrypoints

Extensions via entry points
Better config exception handling

Docker-DCO-1.1-Signed-off-by: Olivier Gambier <olivier@docker.com> (github: dmp42)
Slight grammar thing

Docker-DCO-1.1-Signed-off-by: Olivier Gambier <olivier@docker.com> (github: dmp42)
Fix #521

Docker-DCO-1.1-Signed-off-by: Olivier Gambier <olivier@docker.com> (github: dmp42)
Fix test

Fixed broken path resolution

Docker-DCO-1.1-Signed-off-by: Olivier Gambier <olivier@docker.com> (github: dmp42)
Enhance tag error output

Docker-DCO-1.1-Signed-off-by: Olivier Gambier <olivier@docker.com> (github: dmp42)
Cleaner bugsnag abstraction

Docker-DCO-1.1-Signed-off-by: Olivier Gambier <olivier@docker.com> (github: dmp42)
Enhanced debugging infos

- removed version / config leakage in headers for all requests
- moved versions output to ping
- added more hosts debugging infos in the ping endpoint
- renamed _versions to _debug
- removed version from the / endpoint
- made / text configurable (issue)

Docker-DCO-1.1-Signed-off-by: Olivier Gambier <olivier@docker.com> (github: dmp42)
Remove unused status endpoint

Docker-DCO-1.1-Signed-off-by: Olivier Gambier <olivier@docker.com> (github: dmp42)
Move bugsnag and newrelic code

... away from the main source to the extras subfolder

Docker-DCO-1.1-Signed-off-by: Olivier Gambier <olivier@docker.com> (github: dmp42)
Re-express CORS as extra

Docker-DCO-1.1-Signed-off-by: Olivier Gambier <olivier@docker.com> (github: dmp42)
Enhance documentation

Docker-DCO-1.1-Signed-off-by: Olivier Gambier <olivier@docker.com> (github: dmp42)
Better bugsnag exception handling

Docker-DCO-1.1-Signed-off-by: Olivier Gambier <olivier@docker.com> (github: dmp42)