Add support for Healthchecks

[fubarhouse]

This PR resolves #269.

This will add some nice outputs in docker ps to indicate if each container passes a given health check.
These checks are supported via official Docker API's and the PR includes all of this and testing.

• DNSMasq: Will check that the process is actually running inside the container.
• HAProxy: Will check if the stats route can be successfully fetched.
• MailHog: Will check if the MailHog route can be successfully fetched.
• SSH Agent: Will check if $SSH_AUTH_SOCK is not empty.

These checks will be run every 30 seconds, will retry 5 times in the event of a failure (2.5 minutes total), start after 5 seconds of container running and the checks will time out after 5 seconds of no response / command execution completion.

18bd6e5e1e6b        amazeeio/ssh-agent       "/run.sh ssh-agent"      23 minutes ago      Up 22 minutes (healthy)                                                amazeeio-ssh-agent
34dfa62c2fed        mailhog/mailhog          "MailHog"                23 minutes ago      Up 23 minutes (healthy)   80/tcp, 8025/tcp, 0.0.0.0:1025->1025/tcp     amazeeio-mailhog
6e2e0fb28db2        amazeeio/haproxy         "/app/docker-entrypo…"   23 minutes ago      Up 23 minutes (healthy)   0.0.0.0:80->80/tcp, 0.0.0.0:443->443/tcp     amazeeio-haproxy
e16d2bcc35cc        andyshinn/dnsmasq:2.78   "dnsmasq -k -A /dock…"   23 minutes ago      Up 23 minutes (healthy)   0.0.0.0:6053->53/tcp, 0.0.0.0:6053->53/udp   amazeeio-dnsmasq