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

Object.factory returns user defined types if exists. Also returns same type for two calls with same arguments.

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.

made skip and limit consistent with other query methods

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

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

it supports setup.py develop

Added README and unit tests on Push functionality. Changed functionality slightly to allow channels= to be specified alongside a where= argument.

convert Date to iso8601 with 3 digits microseconds

Fixing buggy interaction with Cloud Code

Set up cloudcode directory as package_data so it would be included in tests not run in the downloaded directory. Also set up a tests.run_tests() function so that one wouldn't have to download setup.py to run tests, and modified README. Fixes #37

Added informative error message for 'Cannot filter for a constraint after filtering for a specific value': e.g. filter(score=20).filter(score__lt=30).

Add, Remove, and query Relations

Add emailVerified to protected_attributes

Without this change, you will not be able to save users with getting an error code":105,"error":"the emailVerified key is a reserved word.

Update datatypes to check for circular pointer references. Change circul...

Update datatypes to check for circular pointer references. Change circular pointer references so that they return a string instead of pointer object for the secondary reference

Relational queries weren't possible because as_pointer was False from the Queryset, fixed and added test for relational query. Updated PyPI version.

Python3 support

Add master key header only if no user session is specified