Added WithinDelta checker

Based on Nathan Youngman's http://nathany.com/ work
found here:
https://github.com/nathany/go-poodr/blob/master/chapter9/gear1/gear1_check_test.go
and discussed along with other golang testing things here:
http://nathany.com/go-testing-toolbox/

Distributed under the Simplified BSD license found here:
https://github.com/nathany/go-poodr/blob/master/LICENSE