allow configuring ::DateTime::GMTIME

Having to modify application's code to configure whether to use
`localtime` or `gmtime` is sub-optimal. This patch adds a top-level
configuration option, `utcDateTimes`, to allow choosing which one to
use from the configuration source.

The other commit removes a useless `use` line that produced a circular
dependency that prevented `::PatternLayout` from being loaded on its
own (for subclassing, for example).