AppVeyor integration

[stil]

This pull request makes several improvements to build process to make it work with AppVeyor automatic builds.

• Improve code formatting in Python build scripts
• Fix broken call in NuGet/build.py, changed helpers.emptydir('./build') to Helper.emptydir('./build')
• Add method that installs missing pacman dependencies in MSYS2 (Helper.py:Helper.install_pacman_deps())
• Prepend PATH instead of appending it in GtkSharp.py. This way /c/Windows/Microsoft.NET/Framework/v4.0.30319//csc.exe is used instead of /c/Program Files (x86)/MSBuild/14.0/Bin/csc.exe. That fixes a compilation bug caused by whitespaces in Program Files path.
• $(WIX) variable was configured but never used. I introducted $(WIX_CANDLE) and $(WIX_LIGHT) variables which eliminates potential errors in AppVeyor.

I confirmed following minimal AppVeyor configuration being able to automatically compile gtk-sharp

version: 1.0.{build}
environment:
  PIP: C:\Python35-x64\Scripts\pip.exe
install:
- ps: >-
    $env:Path = "C:\msys64\usr\bin;$($env:Path)" # https://github.com/appveyor/ci/issues/597#issuecomment-196849776

    & $env:PIP install executor yattag vsgen
build_script:
- ps: >-
    cd C:\projects\gtk-sharp\NuGet

    C:\Python35-x64\python.exe build.py all
artifacts:
- path: NuGet\nupkg\*.nupkg

Let me know if you're willing setting up auto-builds in within openmedicus/gtk-sharp repo. Please feel free to discuss this pull request.

AppVeyor is able to automatically build nupkgs every commit, which is what we're missing right now.

I'd suggest squashing this pull request into one commit if it's going to be accepted.

[harry-cpp]

Side note, in case auto building of NuGet does get accepted, I can give you access to https://www.nuget.org/packages/gtk-sharp3/ so you can setup auto publishing there :)

[mikkeljohnsen]

How do I setup auto building ?

[stil]

Can you chat on IRC/Hangouts or other IM? I'd quickly explain what you need to do. Basically, you need to sign up on AppVeyor.com, add your project openmedicus/gtk-sharp and do a little bit of setup to ensure it works.

[harry-cpp]

@stil @mikkeljohnsen What are your nuget.org usernames?

[stil]

@mikkeljohnsen invited you to gitter chat if you want to discuss. Link: https://gitter.im/gtk-sharp/Lobby