Validate twitter agent credentials

Added variable caching and validation of credentials. Would love to get the specs passing but for some reason I'm getting a NoMethodError on allow_any_instance_of which is bizarre is rspec 2.14 has this available. Open to other ways of doing this or advice if I'm doing something wrong. 
I don't think this is too expensive and if I don't do this it could break people's builds when the update if they have other specs for twitter agents.