Add lambda support for namespacing

This just adds lambda functionality to namespaces so you can do something like:

```ruby
cache.namespace = lambda { @last_mod_time }  # Set the namespace to a variable
@last_mod_time = Time.now  # Invalidate the entire cache by changing namespace
```

This particular example comes straight from the docs, so I had assumed this was supported. See here: http://api.rubyonrails.org/classes/ActiveSupport/Cache/Store.html.

Let me know if you need any changes.