Caching the local timezone

I was profiling our application that's using DynamoDB, and the creation of the DateTime::TimeZone objects in _init_security_token turns out to be very expensive. This change would potentially cause problems on systems where the local timezone changes, so definitely let me know if you have any concerns. 

This is a pretty major performance improvement in my application, so if you're hesitant to potentially break people on systems with changing local timezones, maybe I could make this a configuration option instead of caching by default?