msvc precompiled headers

[Peter Kümmel]

Now it's possible to compile without precompiled headers,
from README.cmake:

      - for the Debug and Release build all precompiled headers are enabled
        to compile without pch (to check if all necessary headers are included)
          * use MinSizeRel which only precompiles the STL and Boost headers
          * use RelWithDebInfo which does not use any precompiled headers