Entity::errors() will append errors and will merge the new given errors
with the previous ones already stored in the Entity's $_errors variable.

Refactoring the Entity::errors() proposed appendable behaviour.
Fixed lithium\data\entity\Document test where was expected the old
behaviour.

Adding a reset trigger when $field is set to `false`.

Code cleaning and major Model::validates() test to illustrate a
situation where this feature is useful.

Missed a tip on github, in the case where `$field` and `$value`is given,
array_merge is enough because it's not needed recursivity.