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 think these (here and below) should be `.present?` instead of `.nil?` since an empty string also isn't valid.