Possibility to use IPythom.embed() on workers

Follow-up on our discussion with @mrocklin: It would be nice to use `IPython.embed()` directly in a worker for debugging purposes (in a cluster running locally). Example usage:

```python
from distributed import Client

def logic_to_run_on_worker():
# ... imagine complex logic here ...
# local embed for debugging
import IPython
IPython.embed()

client = Client("tcp://10.128.4.209:8786")
client.run(logic_to_run_on_worker)
```

This currently fails with

```
Traceback (most recent call last):
File "dask_test.py", line 9, in
client.run(logic_to_run_on_worker)
File "/home/PHI-TPS/fkeller/.virtualenvs/dask/local/lib/python2.7/site-packages/distributed/client.py", line 1819, in run
return self.sync(self._run, function, *args, **kwargs)
File "/home/PHI-TPS/fkeller/.virtualenvs/dask/local/lib/python2.7/site-packages/distributed/client.py", line 559, in sync
return sync(self.loop, func, *args, **kwargs)
File "/home/PHI-TPS/fkeller/.virtualenvs/dask/local/lib/python2.7/site-packages/distributed/utils.py", line 248, in sync
six.reraise(*error[0])
File "/home/PHI-TPS/fkeller/.virtualenvs/dask/local/lib/python2.7/site-packages/distributed/utils.py", line 236, in f
result[0] = yield make_coro()
File "/home/PHI-TPS/fkeller/.virtualenvs/dask/local/lib/python2.7/site-packages/tornado/gen.py", line 1055, in run
value = future.result()
File "/home/PHI-TPS/fkeller/.virtualenvs/dask/local/lib/python2.7/site-packages/tornado/concurrent.py", line 238, in result
raise_exc_info(self._exc_info)
File "/home/PHI-TPS/fkeller/.virtualenvs/dask/local/lib/python2.7/site-packages/tornado/gen.py", line 1069, in run
yielded = self.gen.send(value)
File "/home/PHI-TPS/fkeller/.virtualenvs/dask/local/lib/python2.7/site-packages/distributed/client.py", line 1773, in _run
six.reraise(*clean_exception(**resp))
File "dask_test.py", line 6, in logic_to_run_on_worker
IPython.embed()
File "/home/PHI-TPS/fkeller/.virtualenvs/dask/local/lib/python2.7/site-packages/IPython/terminal/embed.py", line 383, in embed
frame.f_code.co_filename, frame.f_lineno), **kwargs)
File "/home/PHI-TPS/fkeller/.virtualenvs/dask/local/lib/python2.7/site-packages/traitlets/config/configurable.py", line 412, in instance
inst = cls(*args, **kwargs)
File "/home/PHI-TPS/fkeller/.virtualenvs/dask/local/lib/python2.7/site-packages/IPython/terminal/embed.py", line 158, in __init__
super(InteractiveShellEmbed,self).__init__(**kw)
File "/home/PHI-TPS/fkeller/.virtualenvs/dask/local/lib/python2.7/site-packages/IPython/terminal/interactiveshell.py", line 439, in __init__
self.init_prompt_toolkit_cli()
File "/home/PHI-TPS/fkeller/.virtualenvs/dask/local/lib/python2.7/site-packages/IPython/terminal/interactiveshell.py", line 274, in init_prompt_toolkit_cli
output=create_output(true_color=self.true_color))
File "/home/PHI-TPS/fkeller/.virtualenvs/dask/local/lib/python2.7/site-packages/prompt_toolkit/interface.py", line 81, in __init__
self.input = input or StdinInput(sys.stdin)
File "/home/PHI-TPS/fkeller/.virtualenvs/dask/local/lib/python2.7/site-packages/prompt_toolkit/input.py", line 67, in __init__
assert self.stdin.isatty()
AssertionError
```

Maybe only a minor change is necessary to make this work and it could be optionally enabled via the Dask config.