WIP: Sphinx Docs for Vector module

Adds Sphinx docs for the new sympy.vector module
In the above you define two different ways of referring to a vector's direction. You have the (mathed) X, Y, Z directions, and the (mathed, with hats) i, j, and k directions. You also talk about the scalars (mathed) x, y, z. I think it may be a good idea before you get into the details of the module to define a vector as something in 3D space, represented as (mathed)

N = x * i_hat + y * j_hat + z * k_hat

And then only refer to the directions as the unit vectors. This also introduces the scalars as lowercase symbols, making it clearer what is meant.