Generate xUnit XML test results

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

CPU Backend: More ops and kernels

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

and remove need for the dso_handle symbol

NGRAPH_CPU_ENABLE

This will be revisited if a performant tanh is needed

cmake now fails if zlib is not found

CPU Backend

Conflicts:
	src/ngraph/codegen/compiler.cpp

the other backend flag

The CPU backend is disabled by default and must be enabled with
the CMake variable NGRAPH_CPU_ENABLE set to true