Upgraded Docker check for Agent 5.1

[LotharSee]

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 Improve Docker events #1094 KeyError: 'from' #1113
• Tag docker.containers.running 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 then fallback, which trigger a Docker 1.2 bug). Fix Problem with Docker 1.2.0 and agent 5.0.0 #1104
• Remove total_* useless metrics
• More pertinent service check
• Much cleaner code and configuration example

[LeoCavaille]

I think most people would assume re.search instead of re.match here. Or maybe just specify this clearly in the documentation?

[LeoCavaille]

Looks good to me! Question for you @LotharSee , how hard do you think it would be to have a docker test suite?

[LotharSee]

Not that hard, but I never played with Travis. But it's definitively something that I'd like to do (and that we SHOULD do).