Base unit test case for service providers

Summary:
To tests our service providers, we provide a generic TestCase
with the following assertion method:

public function assertServiceInstanceOf ($expectedType, $serviceName)

It checks if the type of the specified service from the application
container is an instance of the expected type.

Test Plan:

$this->assertServiceInstanceOf(
    'Keruald\Broker\Broker',
    'broker'
);

Reviewers: dereckson

Subscribers: alken-orin

Differential Revision: http://devcentral.nasqueron.org/D229