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 branch 'fix-format'
rename functions to remove the "datetime_" prefix (already provided by the module name)
add some more test cases

failing decode tests
decode tests that pass, even though they don't conform to the RFCs