add image_ref column to the application table

[gioppoluca]

We need to keep the reference to the image placed in jackrabbit so we need a string column in the application table.

As usual on both Db + java

related to #204

[xeviscc]

Could you also add it into the following script in order to have it up to date: https://github.com/CloudOpting/deploy-cloudopting/blob/master/database/files/postgres_cloudopting_database_initial_script.sql

[cpopsa]

Done