Improve CMake exported targets.

I _think_ this represents some of the "best practices" for exporting
targets. They'll be available in a googletest:: namespace (e.g.
googletest::gmock) with non-namespaced ALIAS targets.

• Added GOOGLETEST_VERSION variable
• Use CMakePackageConfigHelpers, bump minimum CMake version to 2.8.8

Signed-off-by: Dakota Hawkins <dakotahawkins@gmail.com>