Feature/781 Add assertDatabaseHas

[josephmancuso]

Added new assertDatabaseHas which can be used like this:

def setUpFactories(self):
    User.create({
        'name': 'Joe',
        'email': 'user@example.com',
        'password': 'secret'
    })

def test_database_has(self):
    self.assertDatabaseHas('users.email', 'user@example.com')
    self.assertDatabaseNotHas('users.email', 'joe@example.com')

[coveralls]

Coverage increased (+0.04%) to 85.281% when pulling 7b8782d on feature/781 into 9525615 on master.