User-blueprint

Added some helper functions to the user handler.

The blueprint now optionally supports email verification in the registration process:
- The corresponding blueprint route is `/verify/<token>`.
- The `UserHandler` decorators to use on the corresponding application methods to activate the functionality: `verification_email_sender` and `registration_verifier`.
- To stop registered but unverified users from logging in, use the `verification_checker` decorator of `UserHandler`.

Initial release, uploaded to pypi as [`user-blueprint`](https://pypi.org/project/user-blueprint/).