Use DatabaseTasks.fixtures_path instead of FIXTURES_PATH in db.rake

This will allow to set fixtures path in global config easier, it should
be not needed to use FIXTURES_PATH every time user wants to load
fixtures.
Add some documentation to ActiveRecord::Tasks::DatabaseTasks

This is a public class, which could be used directly in order to allow
easier database management. Now it also contains settings, which will be
used by databases.rake.
Don't require Rails.env in order to use AR::Tasks::DatabaseTasks