singleuser-start adjustments

Update Tensorflow example

DockerSpawner sets certain environment variables, so assume jupyterhub entrypoint when defined

ensures common behavior of things like GRANT_SUDO

Fix chown of $NB_USER home

finish removing files from $HOME

`initialize_all_variables` is deprecated, although it still works, and `global_variables_initializer` is the replacement.

- mplimporthook should be obsolete with matplotlib 2.0
- curlrc seems to be obsolete now (updates to base image?)
- set WORKDIR to $HOME instead of $HOME/work (leave work there for compatibility)

Change GID of NB_USER to NB_GID if NB_GID is passed as a parameter

install Julia packages in /opt/julia

reduce files in home directory a bit

singleuser: allow overriding port with JPY_PORT env

Add fonts-liberation requirement, fix #372

Updated patch file to build base-notebook Dockerfile on ppc64le

- add a few missing `--system` flags to conda config
- install default notebook config to /etc/jupyter instead of ~/.jupyter
- add a few missing `conda clean`s

changes for accomomdating lmodern

PR for issue #367

* Mesos issue for https://github.com/jupyter/docker-stacks/issues/330
* Remove jpeg pinning note (it was unpinned a while ago)

Bump to notebook 5.0, jupyterlab 0.18

bumped spark version in the readme to 2.1.0

add jupyterlab 0.17 to minimal-notebook

Adds texlive-xetex package