TimeZoneConverter: move to named captures.

This makes it easier to decipher what matches what and how the defaults
come to be set, in my opinion.
TimezoneConverter: add support for "from" timezone.

We've always been able to convert from a given timezone, but they
weren't allowed to explicitly say so.

The example query exposed a bug in the 'PM' handling for the 12
o'clock hour which is also resolved herein.

Fixes #631.
Merge pull request #632 from mwmiller/tz_conv

TimezoneConverter Updates.