support express.js

hey, i added a bit of code to support the log format that express.js generates with its default logger.
it is mostly an issue with date and timezone.

you probably don't want to pull it verbatim, as the difference between COMMON_LOG_FORMAT and EXPRESS_LOG_FORMAT is just a removed qualifier for the date.

i also had to change the timezone detection as 'GMT' yields a ValueError and i suppose that is fine as timezone = 0.

thanks for piwik!