Add an API to fetch server properties (closes #167).

Support the RPL_ISUPPORT message, allow structured access to the information it
provides, and don't perform anything Banex/Invex-related if the server doesn't
explicitly announce support (via ISUPPORT).

Inspired by a patch written by Thomas Bruderer <apophis@apophis.ch>.
Merge pull request #9 from RavuAlHemio/feature/#167_server_properties