NGRAPH_INSTALL_PREFIX instead of CMAKE_INSTALL_PREFIX

Basic autodiff for +, -, *, /, exp, log, plus numeric testing support

* working unit test currently fails as expected

* unreferenced args unit test pass

* cleanup

* cleanup

Multiply and Add numeric/autodiff unit tests pass

Documentation for all ops that have been defined so far. Includes numerous edits by @ECouzens.

* 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.

* Elementwise sign operation

* Formatting

remove unused file call_frame

add sha1 of eigen download file so it does not have to redownload if …

Merge branch 'cyphers/autodiff' of https://github.com/NervanaSystems/private-ngraph-cpp into cyphers/autodiff