Better exclusion of outer face.

The "outer face", that is, the face that consists of all outer walls,
has been excluded until now by virtue of the safety_counter - basically
it'll have more walls than the maximum per cell and is thus excluded.
That's an ugly hack.  I left the safety counter in for safety, but now
the exclusion happens because the walls on that face are
counter-clockwise since the face is really "inside out".
Added simple point drawing function.
Added "round" maze style.

In this style, xsize is the number of "rings" and ysize is the density.
The density divided by 2 is the number of cells on the inner ring.  As
rings are added to the maze, the density is doubled whenever the arc
length of the outer wall is more than double the arc length of the
outer wall of the first ring.

It's likely that there's a better way to do that with a Fibonacci
sequence, I just haven't given it much thought yet.
Added "round" maze to documentation.