Allow mime interceptor to be configured with a custom mime registry

The 'rest/mime/registry' can now be extended to create child registries.
The child registry can register converters locally, and lookup converters
from its parent.

    parent = require('rest/mime/registry');
    child = parent.child();

The mime interceptor now accepts a 'registry' config attribute. When
specificed, the provided registry will be used instead of the default,
root registry.

Minor changes:
- unknown request Content-Type results in the request being rejected with error 'unknown-mime'
- unknown response Content-Type falls back to 'text/plain'
- all converters returned by the registry are promises
- refined request phase rejected promise, value of promise is now response.error

Issue: #26