If a domain only has an mx record but no A record it
will trigger a ConnectionError when attempting the
nicknym key lookup.

We need to detect and handle this in the http adapter
already because once the exception is handled by Celluloid
our actor will be terminated.

So now we allow for handing a rescue option to the
adapter with a string that is checked for inclusion in
the error message. If the string is found the exception
will be caught and the adapter returns nil.

We only make use of this when checking the availability
of nicknym so far. That should be the only http request
going out.

This is a first step. In case the suspected nicknym server cannot be
reached we will now move on and try other sources.

It's robably not what we want in the long run. In order to know wether
no key exists or we just failed to connect to some servers a different
http response code would be nice if network errors occured.

This simplifies testing such scenarios in the unit test and makes the
remote tests skip on network failure.

Also changed Nickserver::Response to not include the status code.
This may be okay for error responses but in most cases we want to
have a parsable message and not some status code prepended to it.

I added a RequestHandler that uses Nicknym::Source - but that has not
been implemented yet.