add configurable namespace and remove callback

This adds a configurable namespace and removes the callback for set_id. Instead it is called directly in the save method without a callback.

ex:

```` 
class User < CouchRestAdapter::Base
  self.namespace = 'something_else'
end

u.new email: 'test@test.com'
u.save

# u will have _id : 'something_else/test@test.com'
````