Format datetime fields with time and timezone information when present

[Wigny]

Ensures the time and timezone information for datetime fields is included in the OFX when present, accordingly to the spec.

It hides time and timezone when they are missing for fields that were previously rendered as dates (e.g., DTPOSTED), but keeps rendering time for fields that were already rendered with it (e.g., DTSERVER) to avoid breaking changes.

[coveralls]

coverage: 95.22% (+0.1%) from 95.115%
when pulling b2a36c3 on Wigny:format-datetime
into 64073d5 on kedder:master.

[kedder]

Looks good, thanks!