Add leaves scope

Hi. I'd like to use acts_as_tree in a project and one thing I would like in particular is an AR scope for the leaves of the tree, but it was unfortunately missing. This patch adds it. ~~It also converts `#roots` from a method to a scope to allow chaining.~~

~~Running the tests also complained that MiniTest::Unit::TestCase was renamed to MiniTest::Test.~~ (I didn't  `bundle exec rake`...)

I'm not sure if scopes are available in all AR versions acts_as_tree wants to support. Sorry if it's wrong to use them - I can fix that.