Adds support for sorting nodes.

In the array that displays the nodes, headers can be clicked on to
order the nodes by their status, name or report date (the default).

This is done by adding two parameters to the HTTP query:
 * 'o' for order (1 DESC, 0 ASC)
 * 's' for the attribute we want to sort on (status, name or reported_at)

The sort logic is added both
 * To the SQL query when possible
 * To the library when retrieving the nodes by group. In that case
   the nodes are fetch using the graph of groups injeritance
   and then sorted using the HTTP query parameters

This is diplayed on the node list on the dashboard by an arrow pointing up or down before the hearder name ('St' which has been added to the first column for 'status', 'Node' or 'Latest report').