add options; implement strict_missing

(upstream of https://github.com/venmo/btnamespace/pull/3)

assign: @amygdalama 

Currently, non-namespaced resources created before entering a namespace are accessible. ie:
  1. create a resource 'foo'
  2. activate a namespace
  3. look up resource 'foo'. btnamespace will log a warning that it doesn't recognize this id, but let the request continue through to the client library

This adds an option to shortcircuit in step 3 and immediately raise `braintree.exceptions.NotFoundError` (what the client library raises on a 404).

I have it off by default right now, but we might want to make it the default behavior.