Adds ability to set defaults for subsequent serve calls

Hey, nice work here! I added the ability to set a default options object so you don't have to pass in the same options in. For example:

erm.defaults({version: '/v0', lowercase: true})
erm.serve(app, User)
erm.serve(app, Pet, {lowercase: false})

now all routes setup will be of the form
/api/v0/users
/api/v0/Pets