Add GTEST_OUTPUT environment variable definition, to generate an xml report from the unit-testing (written in Google Test).

* Refactored to run builds as user, not as root.  Added make install.  Added parallel build.  Expanded make clean to remove BUILD directory.  Moved log generation to makefile, so Jenkins job can let make do all of the work.

* Changes/fixes to address code review.  Mainly adding double-quotes around variables and using $(...) expansion where appropriate (and semantically identical to back-quotes).

* Add double-quotes for variable expansion safety.  Write error message in run_as_user.sh to stderr.

* Remove chown_files.sh, made obsolete by running as user inside docker container.  Add set -e and set -u to run_as_user.sh, for further protection.

Moved all apt-get install commands into same RUN line as apt-get update.  There is a subtle timebomb if apt-get update is performed in a separate RUN line, due to how docker layers work. (#179)

* Forces CI to use clang-3.9

Changes to allow build (in docker run) to run as the user, not root, so BUILD directory is easily removable.

Added "make check_cpu" target.  Removed unused definitions from Makefile.  Added chown_files.sh to ngraph_cpp_cpu image, instead of creating new image.

Add a couple scripts from ngraph/contrib/docker.  chown_files.sh solves a file permissions problem involving mounted directories in docker images.