properly document the current :date and :date_time converters supported formats

[colinsurprenant]

relates to #48

the current :date and :date_time converters supported formats is not documented. It relies on the Ruby CSV Library which says:

:date
Converts any field Date::parse() accepts.

:date_time
Converts any field DateTime::parse() accepts.

Now both Date::parse and DateTime::parse supported formats are not obvious to figure out either.

This should be sorted out and properly documented and related tests/specs added.