Dynamic scope names

With these changes you can specify a proc for the name shown on index pages for scopes.

Using this syntax:
```ruby
# Assuming you've got a `published_today` scope defined in your model
scope ->{ Date.today.strftime '%A' }, :published_today
```

You can get something like this:
![dynamic_scopes](https://f.cloud.github.com/assets/688886/299493/a1d9a66a-957c-11e2-8e8b-5b34f21e1bf3.png)
(Friday and Thursday are dynamically named)