Python3 support
test improvements - added simple test for select related, optimized number of queries in tests
made skip and limit consistent with other query methods
independent derived queries - filter, limit, skip, select_related methods returns queryset copy instead of self

This is proper implementation of idea suggested by orignal limit and skip methods based on metaclass (but not working at all)
It works same as Django queries - original unfiltered query can be still executed or used to create
different filter.
avoid executing empty batch, it has no effect
lazy loaded objects
Object.factory returns user defined types if exists. Also returns same type for two calls with same arguments.
python 2.x fix for Object.factory
Merge pull request #48 from farin/master

Python3, tests for new features, small fix for __len__ and query copying