nGraph v0.6.0-rc.0

Statically link cpu backend into ngraph shared library (#1444)

* static link cpu library to ngraph

* remove debug

* link ngraph and cpu backend into a single shared object

* add -fPIC and whole-archive for CPU backend

* Added conditional for --whole-archive for Mac OS.

* Added more conditonal for MacOS.

* fix linking problem and unit test failures caused by multiple copies of the same function in CPU backend and INTERPRETER

* fix nbench build

* add nbench to unit test build

* add version number to libngraph