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: improve description of shared methods

The description is used by client SDK code-generators like
loopback-angularjs.
Merge pull request #96 from strongloop/feature/remember-scope-target-class

Remember scope target class, improved description of scope methods