Move auth to a separate package

[proppy]

Something like gcloud.auth would allow every service subpackage to reuse the same credentials.

[proppy]

Something like:

credentials = gcloud.auth() # will do the right thing according the the environment or from the args
connection = gcloud.storage.get_connection(credentials)

[dhermes]

This is already occurring in
https://github.com/GoogleCloudPlatform/gcloud-python/blob/master/gcloud/credentials.py

@proppy please reopen if you think this is the wrong a abstraction