better formatting of makeLinesEq answer

Two minor changes to lines in the form ax+by+c=0:

* Common factors of a, b and c are divided away so that the equation is in the lowest terms
* The coefficient of y is always positive
* Slightly cleaner code
added equation of circle questions

This commit adds questions of the form “Find the equation of the circle with centre ... and radius ....” and “Find the centre and radius of the circle with equation ...”. It includes the unit tests.

It also adds a new helper file: geometry.js. Currently this includes functions that take the centre and radius of a circle, and return the equation in the form (x-a)^2+(y-b)^2=r^2 or x^2-2ax+y^2-2bx=C.