* Tests show what was expected and what was actually returned. This both
  helps the students to understand what was wrong, and is more like what
  they'll encounter in real life from test suites.
* Fix up partial rename from getRemainder to getTotal.
* Fix up real-world logic of currency conversion (a % fee should _cost_
  you more, not _give_ you more!)
* Magic 8 ball doesn't use a global variable, and explains slightly more
  what checkAnswer should actually do.
* Magic 8 ball tests to make sure more than one value is actually
 returned.

Note that this introduces use of `require` to import `util` - this is a
built-in module in Node, so doesn't add any extra requirements to how
the tests are run, but does introduce an element of syntax we haven't
taught the students. This is only in the test machinery itself, and is
fairly cosmetic, so hopefully won't confuse folks.

* All functions take arguments - artificial examples which don't involve
  taking input make it less clear to students why functions are useful.
* Show expected/actual - this both helps the students debug, and better
  matches what they should see professionally from tests.
* Test that first5 used slice not splice.

* Show expected/actual - this both helps the students debug, and better
  matches what they should see professionally from tests.
* Make question about removing element less misleading (its old wording
  suggests modifying the array, before telling you not to).
* Add decimal edge case.

* Print expected/actual from tests.
* Clarifications in text describing problems.
* Add some edge cases:
  * Oxygen levels - make sure they converted oxygen levels to numbers
    before comparing.
  * Journey planner - Don't have all arrays have the same length.
* Be more specific about what the credit card validator function should
  return.

Improvements to week1 exercises

Improvements to week 3 exercises