MIME4J-298 Convert DateTimeFieldLenientImpl to DateTimeFormatter (#44)

This allows:
 - Specifying all patterns at once, avoiding one parsing pass per pattern
 - DateTimeFormatter is thread safe, thus can be initialized once and reused

Special care have been taken to preserve previous behaviour (missing tests were added):
 - Accept extra input after the date
 - Relax cross-validation
2 files changed