[COOK-4401] Cover existing code with unit tests

https://tickets.opscode.com/browse/COOK-4401

* 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

```
% bundle exec rspec
........

Finished in 2.11 seconds
8 examples, 0 failures

WARNING: ChefSpec Coverage reporting is in beta. Please use with caution.

ChefSpec Coverage report generated at '.coverage/results.json':

  Total Resources:   9
  Touched Resources: 9
  Touch Coverage:    100.0%
```