Asserting an exact number of classes or resources

This adds a few matchers for checking the number of classes or resources in the catalogue.
It allows checking the number of classes or resources, or number of resources of a specific
type (defined types too).
```ruby
it {
     should have_resource_count(2)
     should have_class_count(1)
     should have_file_resource_count(1)
     should have_logrotate__rule_resource_count(3)
}
```

My primary use case is the param class holding OS dependent variables, which should not contain any resources nor include other classes. There is no real existing check for this.