Add --online option to /roster command.

/roster --online will show only the contacts who are online.

Fixes #33.
Print [--flag] instead of <argname> for optinal arguments in /help.
Merge pull request #36 from schulze/master

Print [--flag] instead of <argname> for optinal arguments in /help.
Handle SMPFailed case.

This change causes a message to be printed in the event that an SMP
verification fails.

Fixes #37.
go fmt