convert 12-hour time to 24-hour when returning.

perform some other light normalization on the date time to make parsing
without python-dateutil of mf2util simpler

This addresses issue #42
dt-parsing: infer 00 minutes and 00 seconds

when a datetime has an hour but not minutes/seconds, as
per http://microformats.org/wiki/value-class-pattern#Date_and_time_parsing
Merge pull request #44 from kylewm/am-pm-parsing

Parse dt-* properties for am/pm fixes #42