Add Rotation trait

This will eventually be implemented on Mat3 and Quat
Comment edits
Remove Quaternion trait
Add methods in the Mat3 and Quat anonymous impls in preparation for implementing Rotation

rot::Rotation won't be able to be implemented until https://github.com/mozilla/rust/issues/4306 is fixed.
Complete method implementations
Add component-wise vec/vec multiplication and division methods
Redefine identity and zero methods as wrappers for Matrix::identity and Matrix::zero
Reduce the number of times normalize is called
Made usages of Number::one and Number::zero prettier
Add to_homogeneous conversion method
Remove unnecessary module specification
Change 'Approx' to 'Round' to reflect changes in numeric library
Create separate ToHomogeneous trait
Fix imports
Fix type inference issue
Updates due to changes in numeric-rs
Add vector constants for axis units
Fix tests
Use Matrix::{identity,zero} functions instead of MatN::{identity,zero}
Impl 'new' constructors from VectorN traits