Add test list.

Highlight the test we'll be tackling first.
First failing test: compilation failure.

Use the best possible API for multiplication of the Dollar currency.

Also update the to-do list with things we've added that we don't like.
Fix the compilation errors.

We now have a failing test.
Green bar! Fix the failing test.

Ok, it's a rubbish, very limited implementation, but still. Next step is
to refactor to remove duplication.
Remove duplication of '10'.

This is found in the test and the code, so should be factored out.
Remove more duplication: '5'.

The 5 is provided to the constructor in the test, so we can remove that
from the code by initialising Amount to that value.
Remove yet more duplication: 'Amount'.

Short-hand operators FTW!
And cross off the completed test.
Merge branch 'chapter/1-multi-currency-money'