Split up credentials and QuEraBackend

[weinbe58]

as of #364 we will have fixed issue #290 but the QuEraBackend object still needs those credentials to be instantiated.

@kaihsin proposed to implement an optional function to the batch/task API to update credentials that way when they expire you can retrieve the results without having to save the batch to a file.

we can leave this for later since currently this isn't a very common use-case.

[weinbe58]

Currently I don't think programatic use of credentials really makes sense given the current infrastructure. I will close this for now.