Add Appveyor CI support

[ebraminio]

There is an ongoing Appveyor CI support add on harfbuzz here, don't know for sure but just guessed that can be useful for ots also and it is working, maybe we can upload the result somewhere so OTS binary would be easily accessible for average users without lot of hassle.

[khaledhosny]

Thanks. Uploading the build should be fine as well, no idea how though.

[anthrotype]

You can use Github Releases. You need to create a Github OAuth token and then add a new provider under the "deploy" section in your appveyor.yml setup file.

http://www.appveyor.com/docs/deployment/github

deploy:
  - provider: GitHub
    auth_token:
      secure: <here goes your encrypted github api key>
    draft: false
    prerelease: false
    on:
      appveyor_repo_tag: true       # deploy on tag push only