Updates to be usable in Rails 3.x

[fadzlan]

Changed RAILS_ROOT constant to Rails.root. Unfortunately, this is not compatible with Rails 2.x.

Removed up and down method as well, change it to migrate. This way, up, down and change methods will be called accordingly from the migration files. The reversed logic in change method will also be written to the files.

Let me know if you need this to be compatible with Rails 2.x as well, and I can probably added few conditionals to detect the environments and probably added the up and down methods back in. Otherwise, those who are wishing to use it with Rails 2.x can fall back to use the previous version.