Have ToHomogeneous use &self in method signature

I find makes the syntax a bit lighter.
Merge pull request #35 from aepsil0n/to_homogeneous_self

Have ToHomogeneous use &self in method signature
Add metadatas for crates.io

cc #37.
Implement assertion macros for ApproxEq

These macros yield readable error messages as test assertions for ApproxEq
types. They can be invoked as:

    assert_approx_eq!(a, b);
    assert_approx_eq_eps!(a, b, eps);

Fixes #40.