[SPARK-4065] Add check for IPython on Windows

This issue employs logic similar to the bash launcher (pyspark) to check
if IPTYHON=1, and if so launch ipython with options in IPYTHON_OPTS.
This fix assumes that ipython is available in the system Path, and can
be invoked with a plain "ipython" command.
Removing "with sbt\sbt assembly" is correct. see #2640.