Make security bootstrapping mandatory - closes #250

[benfrancis]

This PR makes security bootstrapping mandatory for:

• Consumers (for all the security schemes they implement)
• Things which require authentication in order to access their Thing Description

Closes #250.

Without these assertions, Consumers will only be out-of-the-box interoperable with Thing Descriptions which don't require authentication, which I expect will be a minority of Thing Descriptions.

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[mlagally]

Arch call on Nov 3rd. Agree to merge after resolving merge conflict