Improved the contribution guide
Merge pull request #251 from GrahamCampbell/2.0-contrib

[2.0] Improved The Contribution Guide
Allow generators to be passed into $attr array

This was actually a bug in the current system that I just happened to
come across here. We could not pass generators into replacement
attributes array when calling ::create ::instance
Add tests around replacement generators
Merge pull request #254 from scottrobertson/attributes-fix

[2.0] Attributes Fix
Shift the target to 2.1
Missed a bit
Throw ClassNotFoundException when a class cannot be found when creating Factory instance
Helper methods for getting group and model without group
Combine group attributes and none group when creating factory

When we create a factory with a group name passed in, we combine any
attributes set on the group factory definition with the base factory
(not a group one). This means we can use the group factories to
overwrite the base factory
Test that group definitions are working
Build attributes array before calling attributesFor
Add tests around replacement generators

A
Make sure we are replacing generators in group factories
Change ClassNotFoundException to ModelNotFoundException