Add cache_key class method to Forest::ApplicationRecord
Add cache_key class method to Forest::ApplicationRecord

You can use this cache key if you want to cache something that is
dependent on all records, or cache something that should be expired
when any record is changed.

```
<% cache [Client.cache_key, 'client_list'], expires_in: 1.day do %>
	<%# a list of all clients... %>
<% end %>
```

dylanfisher/forest,dylanfisher/forest,dylanfisher/forest