Release 4 of the FreeOrionSDK

v4 - 2016-02-17

Don't build python against any potentially installed instance of OpenSSL
to avoid missing dependencies on distribution.
Update libpng version to 1.6.27
Fix Visual Studio target platform SDK selection for boost to allow toolsets
without _xp suffix.
Enable abbreviated pathed in boost build to avoid failed to write output file
with Windows build.
Add Dockerfile to create TravisCI build environment for Linux.
Add patches from Boost.GIL upstream to avoid compiler errors with MacOSX
clang.