User models + admin

I made some changes to the structure for User and Person. I took out Contributor because I realized it was redundant. A User now must be linked to a Person, but a Person doesn't have to be a User, and a Person can have any number of ContributorRoles. 

![user-person-contrib](https://cloud.githubusercontent.com/assets/2547035/5417354/4aeb255e-81ed-11e4-8901-f9204e29327d.png)