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

Add config object

Enable Session Tokens to be used to access ACL-protected objects.

Solve Datetime object not JSON serializable

ACL patch

Added Installation channel subscription modification

Handle adding and removing relations by User.

Added Installation channel modification so that account holders can manually add/remove subscriptions of users on their application

Simple fix using "default=" argument of json.dumps to pass a date_handler.