pretty_print_object

This pull request adds some common object utilities (`__str__` and `__repr__`) allowing the W1ThermSensor object to be nicely printed, providing a more meaningful output than the existing default.

For example, the output from this snippet:
```python
s1 = W1ThermSensor()
print(s1)
```
will change from:
```
<w1thermsensor.W1ThermSensor object at 0xb6b26fd0>
```
to this:
```
W1ThermSensor(name='DS18B20', type=40(0x28), id='000003e9d591')
```