New "link" type for datagrid column format

As proposed here http://www.orocrm.com/forums/topic/datagrid-view-action-as-real-link this is an additional datagrid formatter property to enable rendering columns as link.

First idea was to use the defined urls for view_link action etc, but as there exists a property for "url" I decided to just enhance it as this is the easiest approach and independent of defined grid actions.

We/you might want to add some more configuration. e.g. to be able to add some attributes to a-tag like a class or so, but I wanted to leave that decision up to you guys :)

Also I did not find any unit tests for existing properties, so I did not implement them as well...