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.
Merge pull request #43 from aepsil0n/assert_approx_eq_macros

Implement assertion macros for ApproxEq