experimentally add sort function on query set.
experimentally add sort function on query set.

dosort(field, sort_func, ascending)

Usage:

   XXModel:filter({ foo .. bar ... }):dosort( 'username' )

Signed-off-by: Daogang Tang <6435653d2db08e7863743a16df53ecbb301fb4b0@gmail.com>

daogangtang/bamboo,daogangtang/bamboo