Clean up cmake script and code includes (#106)

* Clean up CMake scripts and related include paths.
* Add unittests.

cmake/googletest-download.cmake

@@ -0,0 +1,20 @@
+# code copied from https://crascit.com/2015/07/25/cmake-gtest/
+cmake_minimum_required(VERSION 3.5 FATAL_ERROR)
+
+project(googletest-download NONE)
+
+include(ExternalProject)
+
+ExternalProject_Add(
+        googletest
+        SOURCE_DIR "@GOOGLETEST_DOWNLOAD_ROOT@/googletest-src"
+        BINARY_DIR "@GOOGLETEST_DOWNLOAD_ROOT@/googletest-build"
+        GIT_REPOSITORY
+        https://github.com/google/googletest.git
+        GIT_TAG
+        release-1.8.0
+        CONFIGURE_COMMAND ""
+        BUILD_COMMAND ""
+        INSTALL_COMMAND ""
+        TEST_COMMAND ""
+)

cmake/googletest.cmake

@@ -0,0 +1,32 @@
+# the following code to fetch googletest
+# is inspired by and adapted after https://crascit.com/2015/07/25/cmake-gtest/
+# download and unpack googletest at configure time
+
+macro(fetch_googletest _download_module_path _download_root)
+    set(GOOGLETEST_DOWNLOAD_ROOT ${_download_root})
+    configure_file(
+            ${_download_module_path}/googletest-download.cmake
+            ${_download_root}/CMakeLists.txt
+            @ONLY
+    )
+    unset(GOOGLETEST_DOWNLOAD_ROOT)
+
+    execute_process(
+            COMMAND
+            "${CMAKE_COMMAND}" -G "${CMAKE_GENERATOR}" .
+            WORKING_DIRECTORY
+            ${_download_root}
+    )
+    execute_process(
+            COMMAND
+            "${CMAKE_COMMAND}" --build .
+            WORKING_DIRECTORY
+            ${_download_root}
+    )
+
+    # adds the targers: gtest, gtest_main, gmock, gmock_main
+    add_subdirectory(
+            ${_download_root}/googletest-src
+            ${_download_root}/googletest-build
+    )
+endmacro()