Improvements to CMake buildsystem

Roll-forward of c90c08cc "Revert "Improvements to CMake buildsystem (#563)""

NEW:

* Replace CMAKE_SOURCE_DIR with CMAKE_CURRENT_SOURCE_DIR

Fixes #612 when yaml-cpp is used as a git submodule.

ORIGINAL:

* Move enable_testing() into proper place

* Added CMake's checks for C++11 standards

Raised minimal version of CMake to 3.1, since on old systems there no
decent compilers that supports c++11.

Closes #377.

* Externalize googletest project

Externalize gtest to avoid installation, fixes #539.

* Remove defined cmake_policies

CMP0012 - OLD marked as deprecated for >=cmake-3.1 and will be removed
CMP0015 - does not affect to build process
CMP0042 - already NEW for >=cmake-3.1

Fixes #505

* Fix compiling in Windows MSVC