Upgraded Docker check for Agent 5.1

Many bugs and feature requests were reported for this check, this PR should address most of them.

* Remove default limitation on the number of containers
* Revamp events: aggregate events by image name, use the body to display more information (names, ids and states of containers). Fix #1094 #1113
* Tag `docker.containers.runnning` by image name
* Remove warning when there is no container running
* Add an option to turn events off
* Add an option to set the socket timeout (useful for users with many many containers)
* Add an option to collect container size (instead of trying first, which trigger a Docker 1.2 bug). Fix #1104
* Remove `total_*` useless metrics
* More pertinent service check
* Much cleaner code and configuration example