Configure Appveyor support

[Julian-O]

MoviePy is currently test using continuous integration using Travis-CI. That is excellent, but Travis-CI only supports Linux and OSX (and is only being deployed with Linux.)

As a result, a number of OS-dependent bugs crept in, and MoviePy did not run on Windows.

PR #626 attempts to address these issues, and hopefully means MoviePy is compatible with Windows now, but to prevent it sliding backwards, it is necessary to add CI testing on Windows.

AppVeyor.com provide a very similar service to Travis-CI.org - CI testing of a matrix of machines based on a YAML script. Like Travis-CI it is free for public GitHub repositories.

#626 includes a working script for AppVeyor that passes (and even runs more code than Travis!) However, an Appveyor account needs to be created and associated with the Zulko/moviepy project and configured to ensure all pull requests and commits are automatically tested, similar to the Travis-CI layer.