Add an updated_date column into the civicrm_contribution table to know when was it last updated

Contributions should have a column to know when was it last updated. Users sometimes need reports based on that parameter, and it's good practice to know when a Contribution changed status.

Attached is a patch that adds this functionality in the Find Contributions page, as well as a corresponding updated_date field to the civicrm_contribution table.

This seems like a good start for a major improvement that could be used in several other tables.