New Dancer::Core::Time to ease time manipulations

This was done after the issue #157 has been reported:
https://github.com/PerlDancer/Dancer2/issues/157

The idea is to provide a common class for that.
another test for forcing the epoch
Fix expressions with +/- prefixes, and dont parse silly strings
Dancer::Core::Cookie now uses Dancer::Core::Time
Fix issue #157 - Session expiration can be set with human readable strings

The Dancer::Core::Session object now uses Dancer::Core::Time to parse its
session duration.

The type of the 'cookie_duration' attribute of
Dancer::Core::Role::SessionFactory has been changed to Str for that reason.
reflect Changes
Merge branch 'topic/time-object'