Adds custom error handling
Users can define a dictionary of what messages and status codes to return when certain exceptions/errors are encountered during a request.
Fixes #221
Adds information about custom error messages to the docs
Adds unit test for custom user messages
Fix import for Python 3 support
Merge pull request #231 from pswaminathan/py3

Fix UserDict import for Python 3 support
Merge pull request #225 from computmaxer/custom_error_handling

Ability to define custom error messages