Point ± Point should produce a Vec?

[stevengj]

Currently, Point – Point produces a Point, which seems wrong for the documented semantics.

[andyferris]

See #102

[andyferris]

I have played around with implementing an Affine space and it is a little tricky to get everything both "correct" and "convenient". If you think Point - Point = Vec then you might want Point + Point = error() and, since Point no longer represents a vector space, you can't have Point being any kind of Union{AbstractVector, FixedVector}.

I would love to see a neat solution for all of this, however.