"date_dt" method for Transaction objects

Add a "date_dt" attribute to Transaction objects to prepare a DateTime
instance of the transaction date if requested.
MooseX::StrictConstructor doesn't import anything

It doesn't work as expected if an empty list is placed on the line
though, so explicitly state as much.
document "mech" Session attribute before others

The other attributes generally aren't touched by anyone, so they're
less important than "mech."
refactor code for clarity
DateTime is an optional dependency

Don't make it a hard dependency when building.
v1.2.0

  - Add a "date_dt" attribute to Transaction objects to prepare a
    DateTime instance of the transaction date if requested.

  - Minor documentation adjustments.