Sorting support in beets query mecanism (dbcore)

Add sorting support  (see #749)

 * in the dbcore API and as a new syntax for queries
 * default sort order can be defined in the configuration file
 * documentation and tests case are update accordingly

Plugin-defined Sort objects are not yet implemented but this could be done later (and everything is in place for that).
This (and the `return` a couple of lines below) can just be:

    return query, sort

No parentheses necessary.