Add callWithResponse (#28)
Fix the error message parser and add TwitterUnknownErrorResponse to TwitterError.

The Twitter documentation says that the error response looks like this:

  {"errors":[{"message":"Sorry, that page does not exist","code":34}]}

However, Twitter occasionally responds error responses, formatted as an undocumented form, such as:

  {"errors":"...."}
Merge remote-tracking branch 'origin/call-with-response'
Merge remote-tracking branch 'origin/fix-parse-error'