Set up CMake build config files. (See extended message)

The project now has CMake configuration files to automate the set up of
build files in a system-agnostic way. As part of this, the GTest source
(and associated CMake files) were also pulled into the build tree.

For examples of how to add include folders, link against libraries,
define output binaries, and declare tests, see the CMakeLists.txt files
in the project root or any submodule subdirectory.