Ddpclient

Upgraded Python 3.6
Upgraded DDP API version to v201710

Change log:

Updated to Google DDP API v201609

Change log:
1. Added `get`, `add`, `remove`, `set` methods to the `Client` class
2. Created concrete selector classes (`UserListSelector` and `UserListClientSelector`)
3. Create Soap instant objects in Client class instead. As a result, `Operation` and selector class does not need a client instance before method `build` is called.

Breaking changes:
1. The `Selector` class has become internal. Concrete classes `UserListSelector` and `UserListClientSelector` should be used instead.
2. Selector classes constructors do not require a client object
3. Client object will build the selector internally. There's no need to call the `build` method on selector objects.

Move credential renewal logic from Auth into Client

1. Add support for passing client customer ID as argument to `Client` constructor
2. You can also use environment variable `DDP_CLIENT_CUSTOMER_ID` to set client customer ID

Add support for custom credentials storage, making storing credentials in database possible.