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
Merge remote-tracking branch 'origin/2.0'
Multiple factory definitions preparation