WIP: Sphinx Docs for Vector module

Adds Sphinx docs for the new sympy.vector module
Talking about the class structure as you are is important, but I'm not sure if it's "introduction" material. As you said, most users will not have to do anything with 'VectorAdd', etc... I'd just talk about the operations that are permitted on vectors. Addition, subtraction, dot, cross, curl, div, etc... You could have a page just devoted to the technical class structure later on, but the intro should really be about what the module is capable of.