root url / subdomain support

[matthewmueller]

It'd be nice to expose something that allows you to modify the root url so you can point to subdomains:

user.save() => POST http://api.example.com/user

[tj]

Try altering User.base that'll default to /user, User.base = '/api/user' etc, though it would be nice to have an api for this at the model level so you can specify a prefix once