Added #pragma for warnings in MSVC warnings

Have added the warnings, as well as made it so that warnings in the external libraries are not activated. Addresses issue <a href="https://github.com/mc-server/MCServer/issues/492">492</href>.

Only able to test it in MSVC 2008 but it should be correct for 2013, but everything is in the required <code>#ifdef _MSC_VER</code>.