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)
Simpler run/wsgi

run:
 * no longer require half of the codebase, it's about starting gunicorn
 * no need for gevent monkey patching (gunicorn gives this)

wsgi:
 * no longer depends on run
 * monkeypatch only if we need it
 * search is now conditionnally inserted, like the other index endpoints
 * debug and log levels are more consistently used

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

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

Docker-DCO-1.1-Signed-off-by: Olivier Gambier <olivier@docker.com> (github: dmp42)
Fix python 2.6 linting

argparse is a third-party library on python 2.6, making hacking to fail.

Docker-DCO-1.1-Signed-off-by: Olivier Gambier <olivier@docker.com> (github: dmp42)
use default new relic environment variables
update gunicorn config in docs
Fix forgotten package

- extras is now included in packages declaration
- alphabetically ordered now to avoid more churn
Grammar and style fixes