Number bases: two new exercises

"Binary Addition" and the more general "Number Base Addition" are included in this pull request.  I'd like to improve the hints, but the current set of hints looks solid and will be sufficient after we have videos for them.  The only files changed rather than added were to tests, to include a test for the new helper class.  If these exercises are accepted, we may want to merge graphie-helpers-numberbases.js with graphie-helpers-arithmetic.js.  If we are going to cover number bases thoroughly, the subtraction, multiplication, and division functions needed for number bases will make graphie-helpers-numberbases.js complicated enough that it will be best in a separate file.