Introduce a safer API

[palazzem]

Overview

In the current state, it's possible to use an HTTP endpoint without any improved security. While this is a use case that needs to be supported, it must not be easy to use it because in most of the cases it will be a mistake using an HTTP endpoint. This issue is proposing to introduce a check that raises an exception if an HTTP endpoint is used:

ElmoClient("http://example.com", "vendor")                 # Raises an exception
ElmoClient("https://example.com", "vendor")                # Is accepted
ElmoClient("http://example.com", "vendor", insecure=True)  # Is accepted

[palazzem]

Adding https://github.com/palazzem/pysettings to validate the HTTPS URL.