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 is a *really* minor consistency issues, but the current beets style uses "full sentences" in comments. That is:

    # Slow sort. Must build the full list first.

with capital letters and periods.