work with self-hosted parse server

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

Added check for config directory in CloudCode directory - fix #83

Revert "Set an ACL on ParseResource by default"

Default ACL breaks some functionalities.

This reverts commit eb437d98.

Closes #97

closes #95

Set an ACL on ParseResource by default

Also switch ACLs default permissions to public read/write

fixes #97

Make ACL default to master key only

Base Role from Object instead of ParseResource

This gives us everything we need from Object and
reduces code duplication

Add the ability to drop whole classes

implments #100

The last 4 unit tests fail when run in series, but not if they were
run individually. The test for testCanGetCurrentUser registers the
service using the session token which loses the previous registration.
The other tests require that they are registered with the master_key,
so now the testCanGetCurrentUser test re-registers.

User.current_user() when a valid session token is provided