supervisor/http.py: Allow request and file gc

Make `tail_f_producer` have weakref to request.
Add `tail_f_producer.__del__` that closes file.

This eliminates 3 `ResourceWarning` warnings mentioned in #391.

### without:

```
$ PYTHONWARNINGS="d" .tox/py33/bin/nosetests -q 2>&1 | grep -c ResourceWarning
13
```

### with:

```
$ PYTHONWARNINGS="d" .tox/py33/bin/nosetests -q 2>&1 | grep -c ResourceWarning
10
```