Remove the need for unnecessary arguments when calling various methods

This removes the need for calling certain methods with unnecessary arguments.

For example, previously you would write:

```python
subscriber = Subscriber(auth, 'listid', 'me@test.com').get('listid', 'me@test.com')
```

Now you can write:

```python
subscriber = Subscriber(auth, 'listid', 'me@test.com').get()
```

Closes #18.