Add chefspec matchers

Add chefspec matchers for users who wants to add unit tests in their cookbooks.

Example in spec.rb files

```ruby
  it 'should create repository' do
    expect(subject).to create_aptly_repo('test')
      .with(distribution: 'ubuntu',
            component: 'main')
  end

  it 'should publish repository' do
    expect(subject).to create_aptly_publish('test')
      .with(type: 'repo',
            prefix: 'ubuntu')
  end

  it 'should publish update repository' do
    expect(subject).to update_aptly_publish('ubuntu')
      .with(type: 'repo',
            prefix: 'ubuntu')
  end
```