Status column in manage listings table

Some column changes in the table of manage listings:

1. Removed country, region and city columns and put together in a new column: location: Since we can't order by any of that columns it seems more readable to put it together in one column
2. Status columns: the first one is to create a color *block* and the second one is to write the status text. For now there are 5 possibilities: spam, blocked, inactive, premium and active.

The row class is set with a new filter: `datatable_listing_class`. This filter receive 3 params:

* class array
* raw row array data
* row array data

Finally, I've created a new helper that was missing: `osc_item_is_enabled`.

![manage listings table](http://f.cl.ly/items/0O1Z0Y2t3L11343w140p/Screen%20Shot%202013-07-09%20at%2012.07.35.png)