Tab completion ignores __all__

The [Python docs](https://docs.python.org/2/reference/simple_stmts.html?highlight=__all__#the-import-statement) describe "public names" as being defined by `__all__` where it exists. It would be helpful if this rule was respected by the Python kernel to avoid module authors that want to provide helpful tab-completion having to obfuscate imports, etc. with leading underscores. NB. This use case is explicitly recognised in the Python docs:

> ... to avoid accidentally exporting items that are not part of the API (such as library modules which were imported and used within the module).

WIth the current kernel behaviour, code such as:

``` python
from __future__ import (division, print_function)
from six.moves import (filter, map)

import contextlib

__all__ = ['my_api']

def my_api():
...
```

needs to become:

``` python
from __future__ import (division as _, print_function as _)
from six.moves import (filter as _filter, map as _map)

import contextlib as _contextlib

__all__ = ['my_api']

def my_api():
...
```