Support endpoint querying via DNS SRV records

To add new SRV endpoints, list then in the configuration at the
srv_endpoints key.

This feature requires pydns (python-dns on a Debian-based system) to
work.
Add some error handling core
Move the DNS query for the SRV endpoiints from FedMsgContext to config.py
Document the srv_endpoints config key.
Merge pull request #165 from laarmen/feature/srv-records

Feature: Fetch endpoints from SRV records