Added healthcheck on Search (abstract) and Solr.
Basic (but working) healthcheck on configured services.

Call a method "healthcheck" on every registered class in the configuration healthchecks. Accessing /_health by HTTP returns a list (in text/plain) of class names and statuses.
Returns 500 if an healthcheck fails.
Refactoring healthchecks in core; returns 500 if no healthchecks configured

Is it the correct behaviour?
Updated solr check to handle errors in a better way.
Updated the configuration to display a "friendly" name instead of the configuration of the service.

This has the advantage to allow easily to test a service multiple times, and does not expose any internal configuration.
Added PING healthcheck for Redis.

The code is specific to Redis (but we only support Redis atm).
Added Cache-Control header for /_health.
Added some documentation.
Added basic test for healthcheck.
Healthcheck Redis: updated comment on PING command.
PEP8 changes