Refactor Authentication model

[benreyn]

There are a number of reasons that we have grown to regret the model of using Partner as the authenticate-able model. There was a conversation in slack about this today.

Ref issues #113, #84

The ultimate solution here (I think) is to add a user model, and restructure the application around a user belonging to a partner, similar to diaperbase's model of having a User belong_to an Organization

cc: @seanmarcia, @chaserx, @armahillo