Adding support for error object Mantle subclass to requests.

When errorResultClass is supplied, Overcoat attempts to instantiate an error object from the response if errors are returned from AFNetworking.
Adding test cases for error objects
Changed API for error objects
Set errorResultClass when initializing instead of the readwrite property
Adding another convenience constructor
Adding test case for errorResultClass after initialization
Merge pull request #31 from alleus/master

Support for server error Mantle objects