Backend subclasses should inherit docstrings from base class

The Backend base class has perfectly nice docstrings, but none of these will show up if you type:

```
help(ln.backend.sql.SQLBackend)
```

Sphinx apparently will pick up on the docstrings from parent classes, but other documentation display tools do not.

There is a recipe for docstring inheritance via a decorator here:
http://code.activestate.com/recipes/576862/
