Support ISO8601 server times

Modify the new server-time feature to support the ISO8601 format as defined by the server-time CAP spec: http://ircv3.atheme.org/extensions/server-time-3.2

Attempt to maintain backwards compatibility with old server-time formats supported in various ZNC releases (including `@t`/`@time` keys, and ISO8601/unix-timestamp values).

Send both the old `znc.in/server-time` and `znc.in/server-time-iso` capabilities since we now support both formats.

This provides for support for bleeding edge ZNC from git, which sends:

    @time=2013-08-18T122:32:15.991Z :brandont!brandont@example.org PRIVMSG Wiz :Hello

and the current ZNC release, version 1.0, which sends:

    @time=1376865135.991 :brandont!brandont@example.org PRIVMSG Wiz :Hello

as well as older ZNC releases, which send:

    @t=1376865135.991 :brandont!brandont@example.org PRIVMSG Wiz :Hello