Improvements to somerby.net/mack/logic

 After a spate of good input from commenters, I've improved somerby.net/mack/logic in several ways:

• Instead of showing just one counterexample, clicking the "Counterexample" button more than once causes the application to cycle through different counterexamples for the current statement.
• There is a new button: the "Example" button, which is the opposite of the "Counterexample" button.
• Propositions (nullary predicates) can now be lower-case letters as well as upper-case letters.
• There is now a "therefore" operator -- ',' -- apostrophe comma apostrophe, for representing logical arguments.