Support Marathon readiness checks

[timoreimann]

Readiness checks are a means to prevent Marathon tasks from being taken into LB rotation prior to them being ready to serve traffic. The idea is that applications define a readiness check HTTP endpoint along with a number of parameters in the Marathon specification, which will cause Marathon to only mark a task as ready during the deployment of the associated application if that endpoint returns a successful response.

Traefik already vendors a copy of the go-marathon that brings readiness check support. The only thing left to do in Traefik is to filter out tasks whose readiness check result is not positive.

Refs #1185.

[timoreimann]

Internally, I have already implemented the necessary logic for Bamboo. It should be fairly trivial to port it to Traefik, which I'm going to do.