Minor DRY refactoring
Media fields returned even in embedded documents.

Trying to retrieve a media field (say an image) by embedding its
content in a related document containing its metadata was not
working properly (only the ObjectID was being returned).

Closes #305.
Make test run under python 3.x (and reinforce it)
Resource links don't take API_VERSION and URL_PREFIX into consideration.

As the HATEOAS links are now relative to API entry point (the root), we
don't want API_VERSION and URL_PREFIX to be included. Since they are
returned by request.path we have to strip them out manually.

CLoses #419.
feat(hateoas) - include blueprints when HATEOAS is enabled.

feat(internal resources) - add the capability to declare some resources as
internal, to be used only for data manipulation.
feat(blueprints) - add_cors_headers()