Change package name from "cgmath-rs" to "cgmath"

When the Cargo package manager gets the online package list component, the package name 
will be used as the identifier for the whole project. Keeping "-rs" -- while useful for the github 
repository -- will be redundant in Cargo because all of the projects will obviously be rust projects.

This naming convention was adopted by one of your other repository gl-rx (https://github.com/bjz/gl-rs/pull/88)