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).
Yes, that’s it; replacing `def sort` with `def __call__`. Does this make sense to you? Because I don’t know how exposed the rest of the `Sort` class’s methods are. Also depends on whether we can imagine passing a simple function as the `sort` argument.