Use 12-hour clock for :en-US and :en-CA

Fixes #335

In addition for time:formats:defaults the time zone as been modified from 'time zone as hour and minute offset from UTC' to 'time zone abbreviation name' to match common US usage as used for example by the US Naval Observatory (http://tycho.usno.navy.mil/timer.html) and the New York Times.
Merge pull request #368 from digitalfrost/patch-335

Use 12-hour clock for :en-US and :en-CA