Allow "reading" all 4 button values at once

The underlying Seesaw class has "bulk reads" for digital outputs, which can be exposed in a more useful manner in this class, instead of invoking the method with the "private" values and/or knowledge of the Seesaw pins.

Proposed method would be something like
```python
def read():
    """Returns an array of boolean (True/False) indicating the state of all the keys"""
```