[bug 820010] Override S.__repr__ in Sphilastic

This nixes those pesky ValueErrors that get kicked up when calling
repr() on a Sphilastic.

Easy way to test this is by opening `./manage.py shell` and doing this:

```
from questions.models import Question
repr(Question.search())
```

Before the fix, you get a traceback and a ValueError. After the fix, you get this:

```
In [1]: from questions.models import Question

In [2]: repr(Question.search())
Out[2]: "<S {'filter': {'term': {'model': 'questions_question'}}, 'fields': ['id']}>"

In [3]: 
```

r?