Fix some bugs in the time format and in the tests' expectations

Reverse the ordering of year & day
Years are either 2 or 4 digits, fixed-field-width
Always use '+0000' as the output TZ
When decoding, TZ offset is inverted
Fix various bugs in tests
Merge pull request #4 from waisbrot/fix-format

Fix some bugs in the time format and in the tests' expectations