Fix #57: Factor 'generate_signed_url' into a free function in 'gcloud.credentials'

Leave a wrapper method in 'gcloud.storage.connection.Connection'.

Fixes #57.
I disagree:  those count-based "design" warnings are basically trying to substitute a hard-wired thumb rule for judgement.  Why is OK to have 15 locals, but not 16?.  I don't mind it if *you* think the method in question needs refactoring, but I don't see the point in having a tool try to second-guess it.