Assertion macros for approximate equality

For sake of readability and consistency with the standard library macros
assert! and assert_eq!, the macros assert_approx_eq! and assert_approx_eq_eps!
have been implemented based on the ApproxEq trait.

Upon failure these macros yield readable error messages including the input
values and the tolerance (epsilon) used to determine their approximate
equality.

Resolves #131.