Added Server.Addr() helper to allow querying of the server's bound address

This can be useful in testing scenarios by allowing you to have the system pick
a random port by passing 0 on options.Port, but also give the user a way to
query the server for what port it actually bound to.
Merge pull request #31 from apcera/add-addr-helper

Added Server.Addr() helper to allow querying of the server's bound address