Made it possible use custom credentials when sending to queues

Sometimes it is not possible to use the namespace connectionstring as the connectionstring for the bus. This could be in a multitenant solution where each tenant is given shared access rights to their input and errorqueue, or when communicating cross namespaces.

This commit makes it possible to configure the bus with credentials for each "sendqueue", in addition to the bus's own inputqueue.

(Most of the Azureservicebus tests are green (I have not a complete configured testsetup locally), and I have added a couple of new tests illustrating the new functionality)