Sphinx Docs for Vector module

Adds Sphinx docs for the new sympy.vector module
I'm not a big fan of this method name. Shouldn't this be a method of a Vector so that you do this:

`Q.position_wrt(N.origin).express(N).to_matrix()`

You need to at least specify that this will compute the position of Q with respect to the origin point of the reference frame you pass into it. But that is sorta treating a coordinate system as a point.

You want to say "what are the coordinates of this point in coordinate system N?" Why not:

`Q.coordinates_in(N)`