Enforce uniqueness of url across STI child classes

Not sure if this is a desirable feature, but I ran into an issue using STI where I needed to be able to enforce the uniqueness of the url across all of the child classes. For example:

```ruby
class Location < ActiveRecord::Base
  acts_as_url :name, :enforce_uniqueness_on_sti_base_class => true
end

class State < Location
end

class City < Location
end

s = State.create!(:name => 'Chihuahua')
c = City.create!(:name => 'Chihuahua')
s.url == 'chihuahua'
c.url == 'chihuahua-1'
```

As you can see, I've added an `:enforce_uniqueness_on_sti_base_class` option which defaults to false. There's a new test covering it, and I updated the README. Let me know if this is something you are interested in merging or if you'd like me to change anything about the implementation.