Flask required only if necessary (#424)

* Example with Flask support

* Only show import error when trying to use Flask

* Re-organize imports

* Move flask_utils next to related module

* Code style

* Change back to incentivizes

* Includes Flask by default

* Project clean up

* Update Rafael Caricio's e-mail address

* Fix conflicts

tests/api/test_errors.py

@@ -1,4 +1,5 @@
 import json
+
 import flask