Sphinx Docs for Vector module

Adds Sphinx docs for the new sympy.vector module
I'm not sure that I'm fond of this class name. Wouldn't this make more sense:

The super class for coordinate systems: `CoordinateSystem`

Subclasses would then read like you say them:

`CartesianCoordinateSystem`
`SphericalCoordinateSystem`
`CylindricalCoordinateSystem`

Also, I see why you wanted to use "Rectangular" instead of Cartesian, i.e. to match the flow of future classes.

What do you think about not abbreviating the class name. I really don't like that...