First take on jobs

First draft of the jobs support in reactor migrations. This code aims to satisfy the issue #5 

For example usages see: 
**(direct code)**
https://github.com/infopark/reactor/blob/42e0bffad3ee55de1764e839683d6be2287990b0/reactor_test/spec/lib/reactor/cm/job_spec.rb

**(migrations)**
https://github.com/infopark/reactor/blob/42e0bffad3ee55de1764e839683d6be2287990b0/reactor_test/cm/migrate/20130623160751_create_test_job.rb
https://github.com/infopark/reactor/blob/42e0bffad3ee55de1764e839683d6be2287990b0/reactor_test/cm/migrate/20130623160809_update_test_job.rb
https://github.com/infopark/reactor/blob/42e0bffad3ee55de1764e839683d6be2287990b0/reactor_test/cm/migrate/20130623160825_delete_test_job.rb