[3274] Changes after review:

 - RelayInfoParser now takes Option::Universe as last parameter;
 - Extra tests for checking invalid parameters in RelayInfoParser
 - Comments in subnet.h improved
 - Sanity checks in RelayInfoParser implemented, doc improved
[3274] Fixed two typos.
[3274] Changes after second round of reviews:

 - getRelayInfo() implemented, relay_ is now protected
 - setRelay => setRelayInfo
 - Option::Universe is passed as const reference
 - Further clarified white_list_ field in Subnet.h
[3274] white_list_ is now of ClientClasses type.