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, we should indeed address this. Post-merge seems fine.

It is nice to have the string API available so that stuff like `Library.albums("beatles")` works (so querying the database, probably the most fundamental operation, is not overly onerous). Maybe we can put together a reasonably balanced solution (later).