Allow znc timezone to be configured

Since 1.0, znc has allowed you to specify the user's timezone:
conveniently, in tzinfo format.  This allows the user to configure and
specify that timezone.

This matters because it affects the timestamps that znc issues when
playing back the buffer after a disconnection.

for more info see:
- http://wiki.znc.in/ChangeLog/1.0#Timezones
- http://wiki.znc.in/Configuration

There is also a zpush_timezone configuration option, which could at some
point be unified with irc_timezone into a common configuration item.