Implementation of a more flexible custom error handler

As discussed in twilio/flask-restful#280,
Code has been adjusted to allow for more flexibility in creating custom error handlers. 
As a result pas addition of custom error handler is no longer required.

Passing a dict to the Api constructor is a great idea but in the end, you still have to define your exceptions somewhere, so if you have to, then you should define those exception attributes into the Exception, not alongside in a dict.