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).
Do you mean adding a `__call__` method on the `Sort` class to make it callable ?