Added more Tests to the Plane class.

Ping @smichr
I really don't like the use of `equal` -- it is too close to `equals`. Don't we have something for testing similarity like `is_similar`? It seems like that would be better for this case. Or better, the points used to define the line should be standardized in some way so the order of points is immaterial (like Segment's handling of points):

```
>>> Segment((1,2),(3,4))
Segment(Point(1, 2), Point(3, 4))
>>> Segment((3,4),(1,2))
Segment(Point(1, 2), Point(3, 4))
```