mathmo questions on circle/line intersections

This pull request finishes the questions discussed in issue 16.

Specifically, it adds:

* A full question and answer for circle and line intersections, where the computer chooses between C/C, L/C and L/L
* Minor changes to the circle equations for prettier printing (736105b)
* A helper function for simplifying two part surds

Please also note the commit message for 27643e7 (specifically, an incorrect topicID which has now been fixed). You might need to clear your question store if you had the circle/line questions loaded.