Add connection name validation

[ptrf]

This patch solves the connection name validation in #39

If accepted, this PR makes turtle fail on attempts to start publishers or services that reference a connection that has not been defined in the turtle connection_config, as described under the Configuration section in the README

[ptrf]

The failing travis-ci build is for Erlang 19.3. There's an issue with the ranch_proxy_protocol dependency, which fails to unpack. I've previously run into this issue, which has to do with erl_tar in 19.3 failing with {error, bad_header} error, when attempting to unpack the ranch_proxy_protocol package.

I have yet to report this issue.

[ptrf]

The erl_tar issue is very weird, but basically stems from a bug (that has already been fixed again) in the hex rebar3 plugin. By upgrading rabbitmq_common, a new version of ranch_proxy_protocol will be pulled in, whose package can be unpacked on erlang 19 :-)

[jlouis]

There is a Changes section in the README.md. Could you also add to that one so we can track the changes we are doing to the system?

[ptrf]

Thanks! Will do