Signed-off-by: NJosé Valim <jose.valim@gmail.com>

Signed-off-by: NJosé Valim <jose.valim@gmail.com>

Some of the engines needs database. As engine needs to be run inside
Rails application, migrations and other database operations are run
from dummy application. To make that process simple I changed db
tasks to look for migrations in both engine's and application's
db/migrate directory. You can run all of the database tasks from
test/dummy or directly from engine with prefix app, like:

  rake app:db:migrate
  rake app:db:migrate:redo

Because AR::Rollback do not reraise and inner transaction is not "real"
nothing rollback at all