Add a setter method to allow changing the timezone used when creating
new log records. This provides a means to correct a behavioral change
introduced by 6cbdc04 where use of the PHP runtime default timezone
provided by `date_default_timezone_get()` was introduced. In most
environments the new default behavior is preferable, but if an
application allows the default timezone to vary between requests it
causes confusing log output.
- constants defined for the 2 missing levels: NOTICE and EMERGENCY.
- add<level>() and <level>() convenience methods added.
- TestHandler and tests updated to account for the two extra levels.
- surjective mappings from the RFC3164 to only 6 levels changes to bijective.
- README updated accordingly.
