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.
Update README.md to reflect item name changes

There were some outdated names presented in the readme. Fixes #128.
Merge pull request #133 from aepsil0n/readme_update

Update README.md to reflect item name changes
Merge pull request #132 from aepsil0n/approx_eq_macros

Assertion macros for approximate equality