Windows build

This pull request contains a working build for Windows and an Appveyor script. The build is successful, but the resulting binary package is not complete yet. Some DLLs on which Graphviz is depended should be included along with the header files, etc. This is something I could work on later if this is appreciated.

Some dependencies were required, like GTK and expat. These had to included on Appveyor, but were not all easy to find or had no automated installation for Appveyor. I included those dependencies as Git submodules.

Getting the build to work is done solely through configuration files and defines, no actual changes in the code were made.