Add operator trait constraints to std::num::{Zero, One} and document their appropriate use

Zero and One have precise definitions in mathematics. Documentation has been added to describe the appropriate uses for these traits and the laws that they should satisfy.

For more information regarding these identities, see the following wikipedia pages:

- http://wikipedia.org/wiki/Additive_identity
- http://wikipedia.org/wiki/Multiplicative_identity
Improve std::num::pow implementation

The implementation has been made more succinct and no longer requires Clone. The coverage of the associated unit test has also been increased to check more combinations of bases, exponents, and expected results.