Create questions
Add hints and solution
Include questions with expression multiplied by a variable (which cancels)
Add functions I've added elsewhere
Remove old exercise
Merge branch 'multi-rat5' of https://github.com/petercollingridge/khan-exercises

Conflicts:
	exercises/multiplying_and_dividing_rational_expressions_3.html
	utils/rational-expressions.js
Can't negate an expression

Test plan: http://localhost:8000/exercises/multiplying_and_dividing_rational_expressions_5.html?debug&problem=combined-top&seed=123

Auditors: peter