Add /renter/contracts route

To enable this, the proto.Contract type had to be moved to modules. This is an interesting side effect of requiring module interfaces. Since proto imports modules, the Renter interface cannot return a proto type. If the interface were not defined in modules, this circular import could be avoided. However, organizationally, I can see the benefit of defining all the module-related types in one place.

The IP field was also renamed to NetAddress for consistency. This will break existing contractor.json compatibility, but this seems excusable since contractor.json has only existed in RCs thus far.