support custom url and servers responses without an ip address

Hi,

These proposed modifications enable noipy to work with ddns servers that use the same protocols but are not run by the official providers and aren't at the standard addresses.  It does this by allowing you to specify a custom url on the command line with a "--url" argument.  The reason someone would want to do this is if they want to run their ddns on one of their own internet servers and have control over their dns records.

I also modified response message parsing to no longer crash in the event that a "good" or "nochg" message is received without an IP address, as is the case with this ddns server implementation: https://github.com/ianloic/dreamhost-ddns.

I included a couple of unit tests and they all pass.

Please let me know if I can be of any assistance,
Jay