Add Entity for default_queue table

I think it would be nice to have entity beside the SQL file, so that we can use this in combination with "orm:schema-tool:update" instead of manually importing SQL. Please review.
Updating readme for newly added entity

Describing how entity is used by user and module.
Adding clone to all DateTime getters and setters
Type hinting to DateTime
Adding null checks for DateTime before cloning
Cleanup

Removing null tests from setters, replacing if's with Ternary Operators, adding spaces within tests.
Removing required brackets that are not required
Merge pull request #60 from tad3j/patch-1

Provide convenient Entity for default_queue table