Assertion descriptions

[elmundio87]

To help speed up fixing of terraform configurations that fail tests, I propose that all methods that raise AssertionErrors should have an optional parameter where you can put a description of the test.

eg.

validator.resources('foo').property('bar').should_match_regex('^([a-z0-9]|\_)*$', 'All characters should be lowercase or an underscore')

Particularly for those that don't speak regex as a first language!