fixing error status codes for authenticateApiRequest

previously we were issuing 400 for all errors that came out of this
method

now we are returning 401 for any error that indicates a problem
with the supplied credentials or token
Merge pull request #102 from stormpath/api-auth-error-codes

fixing error status codes for authenticateApiRequest
Merge branch 'master' into develop

* master:
  split out change log and add links and intro content to readme
Adding NPM flair.
Removing trailing whitespace.
Fixing formatting.
Copying in the Stormpath text.
Formatting.
Fixing wording.