Add run_at support !!DROPS RAILS 2.3 SUPPORT!!

Added the support for scheduling jobs in the future with run_at. In
scalers/base.rb I've used the where on the Delayed Jobs table to get the
pending jobs.

This forces the use of Rails 3.x! So Rails 2.3 support has been dropped in
this commit. If you know the correct way to do this query in a Rails 2.3 and
Rails 3.x way, please inform me.

Also this introduces a middleware component to check for pending jobs, and if
there are any, it will call to scale up the workers.

Signed-off-by: David Westerink <davidakachaos@gmail.com>