Cover existing code with unit tests

* add Gemfile and add all gems required for testing
* configure chef-zero to make work with data_bags and search easier
* configure Berkshelf 3.0
* cover all resources from "users_manage" LWRP with chefspec tests
* create custom matchers for "users_manage" LWRP
Merge pull request #40 from iroller/COOK-4401

[COOK-4401] Cover existing code with unit tests