ReadTheDocs site, User Guide sections

Make extracted spark files root id

base image on ubuntu:xenial-20180228

I can't seem to configure the Jupyter Albaster theme correctly
and will reach out for help later.

Update tensorflow-notebook readme with distributed mode instructions

Add TEST_ONLY_BUILD arg

Fix netcat, pin tornado 4.x for R + zmq issues, update various R libs

Disable datascience-notebook precompilation

Add netcat

Add pyarrow installation

sparklyr add to r-notebook

Netcat (nc) is often used as a tool to test socket server/client, 
and very useful to quickly get started with, f. e., Spark Streaming. 

See #583.

Update to Toree 0.2.0 RC3