Added inIndex method to SearchBuilder

[tobias-kuendig]

This PR enables the user to optionally query a different index than is specified as default in the config file.

[sleimanx2]

Maybe something like this for the test ?

$builder = $this->getBuilder();
$connection = $builder->getConnection();
$builder->inIndex('test');
$this->assertEquals($connection->index,'test')

[tobias-kuendig]

Since connection is a mock we'll have to use mockery's set method to set the value of the index property. This way we can make the assertion, but it isn't really testing the connection class, is it?

[sleimanx2]

true