No breakpoints by default on a line

Instead of including three break points by default on a line, instead have no line break points as the default, and instead enable the user to add break points by double clicking on the line.

![screen shot 2014-09-18 at 1 14 13 pm](https://cloud.githubusercontent.com/assets/1521387/4319088/67d220f4-3f27-11e4-94bd-407e2ec90955.png)

Could we make a utility function for finding the midpoint of two points? We seem to do that quite a bit, so it'll probably remove some repetition.