Merge pull request #5 from quantopian/volumes

DEV: Add support for mounting volumes from the host.
MAINT: Make dockerspawner pip-installable.
MAINT: Add ipython to requrements.txt.

Just installing jupyterhub via `pip install` can't get IPython until 3.0
is released on PyPI.

See https://github.com/jupyter/jupyterhub/issues/92.