pout method to set sys.settrace

https://docs.python.org/3/library/sys.html#sys.settrace (via [How do I print functions as they are called?](https://stackoverflow.com/questions/8315389/how-do-i-print-functions-as-they-are-called))

It would be cool to be able to do something like this:

```python
with pout.trace():
    # do a bunch of stuff
```

and have pout print everything it traced while in the context manager.

