Make version informations consistent between prometheus components

Fix CircleCI flanky go get

fix typo

Update README image shields

*: bump default Go version to 1.6.2

Fix format argument in retrieval test.

discovery: sanitize Consul service discovery

Fix basic issues in marathon SD

This commits simplifies the SD's structure and ensures that all
channel sends are checked against a canceled context.

Fix for #1503

Bring back logging of discarded samples

But only on DEBUG level.

Also, count and report the two cases of out-of-order timestamps on the
one hand and same timestamp but different value on the other hand
separately.

Explicit link to full install docs added

build: push to Dockerhub and Quay

vendor: update govalidator

Update CircleCI test docker image

Fix git branch with slash as docker tag

Include date and time in graph hover detail.