scope: add _targetClass to scope property

Store the class of the results returned by a scope find method.
This class is different from the "targetClass" when the relations
is using a third "through" model (e.g. hasAndBelongsToMany).
Re-enable skipped test.

test/relations: hasMany should fetch all scoped instances

The test is passing with the current implementation.
scope: rename methods exposed via remoting

Rename 'get' to 'find' and 'delete' to 'destroyAll' in order too keep
the names consistent with the scope API. These names are used by
client SDK code-generator like loopback-angularjs.