Merge branch 'win'

Ports the strongSwan core libraries and some plugins to the Windows platform
using a MinGW based toolchain. Beside generic platform abstraction and
the windows.[ch] compatibility layer, this merge introduces a Windows native
threading backend and a charon-svc Windows IKE service.

Travis adds a MinGW cross-compile build to Windows, and further enables -Werror
to let builds fail for all compiler warnings with gcc and Clang.