Cleanup tests

Create helpers and matchers to DRY the ChefSpec examples.
Also prefer the new `expect(x).to` over `x.should`.

Fixes #9
Move recipe specs to spec/recipes
Add unit tests for library methods
Update changelog

[ci skip]
Merge pull request #9 from tmatilai/cleanup-tests

Cleanup in the ChefSpec tests and add unit tests for library methods
Drop a digit from Wheezy version number in readme

[ci skip]
Release v1.5.4

Hyvää juhannusta!