added keys() queryset function

Adds additional support for Relations.

add support for jobs end point

Fix error iteration on dictionary python

work with self-hosted parse server

Relations are different from traditional
SQL join tables.

Changed some comments. Rewrote part of the documentation.

Polished code, added comments. Also tested integration
with some external code.

Tests with and without pre-existing schema on
the server.  Cleaned up logic pyramids, eliminated
redundant flag. Found bug in test of inserting object
of incorrect type.

Code coverage revealed some untested paths
related to retrieving rows from Parse,
rehydrating objects, and then using relation(),
and using relation() before it has a type.

Adds a relation() method to objects. It returns a Relation
with methods to add(), remove(), and query() for related
objects.

with MasterKey('key'):

Added automatic conversion of array of pointers to parse object 

Fix to SessionToken __exit__ . Addition of MasterKey class to have master key removed upon exit of with statement.

Suport regex query-constrains

* https://www.parse.com/docs/rest/guide/#queries-constraints

workaround for #113

Added Remove Field support on Object, including tests

Removed extra line after `testCanRemoveField` test case

The `remove` method works like the `increment` method of the `Object` class, but instead incrementing it clears a column value.

Fixed minor problems caused by tests