Updated docstring on Key.rename() with a warning.

I have a pending request to the GCS team asking if this is a reasonable thing to have exposed, however based on our conversations in #99 and #224 it seems that it needs to exist regardless (if we don't provide it, people will just do this on their own).

That said, we need to be clear about what's happening under the hood in the short part of the docstring. If someone is frequently "renaming" 5TB objects (that's the maximum size currently), this could actually change their bill a lot and make the GCS API do some ridiculous amounts of work. For small objects this shouldn't really be a problem.
Merge pull request #236 from jgeewax/master

Updated docstring on Key.rename() with a warning.
Removing gcloud.credential.Credential class.

No instances of this class were ever created and it only had
one factory method, which is now a function. This class was
also misunderstood / misreferenced in the docstring mentions.
Updating docstring on credentials.get_for_service_account function.
Merge pull request #235 from dhermes/remove-credential-class

Removing gcloud.credential.Credential class.
Merge remote-tracking branch 'upstream/master'
Lose trailing whitespace.