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).
We can get rid of the inline conditionals if `build_sql` returns `query` when it would now return `True` as `slow_query` and `None` otherwise. Same goes for `slow_sort`.