Feature/modeling suppliers

Didn't really write any tests for this feature, as I didn't really know how I wanted to approach the Suppliers and Users relationship. I'm also not sure how to test has_and_belongs_to_many associations, so I just got things working. I would be interested in going back and writing tests for these at some point.
I created suppliers model and views. Will probably switch the logic to associating a supplier to a user, rather than a user to a supplier, but I will wait for feedback from the client.