upgarede to setuptools

it supports setup.py develop
Query.exists() implemented using count, without fetching data. Added __len__
selected_related - Django like method to declare related objects fetched with based query (using 'include' parameter from Parse API)

https://www.parse.com/docs/rest#queries-relational
Merge pull request #47 from farin/master

select_related feature, result cache +  few updates to have more Pythonic code