New first-order examples

[chaudhuri]

Some new examples have been proposed for natural numbers, addition, commutativity, etc. by @lambdacalculator.

[kyagrd]

Not a first order example but I've recently been working on formalizations of Intruder Deduction and Observer Theory and related proofs of some basic lemmas on consistency of the theories, which I can soon contribute as an example when the issue #77 gets resolved :) I think I've hit another bug.

[chaudhuri]

Great! Will look at #77 right away since the CADE deadline got moved by a week.

[amerikan]

@chaudhuri will you consider changing the Github issue label from just text to possibly documentation. I think it would be more meaningful.