Add CI use appveyor

[kunedo]

I wrote an appveyor build script base on your build guide. I use git clone instead of git submodule.

Should change the auth_token in appveyor.yml to yours.

[maxwell-k]

Thank you @kunedo!

I like this a lot. I can see that your build passes at
https://ci.appveyor.com/project/kunedo/ledger-windows-build/.

I have just updated the instructions to use VS2019 but I am happy to move back to 2017, as it is the most recent AppVeyor image https://www.appveyor.com/docs/windows-images-software/. This issue is tracking AppVeyor support for VS2019: appveyor/ci#2907.

I'd like to stick with git submodules, so that this build is more reproducible -
must use specified versions of each repository. I think that git submodule update --init --recursive in the install phase should work as explained in https://www.appveyor.com/docs/how-to/private-git-sub-modules/.

I have some other ideas about how to keep this in sync with the instructions in README.rst, which I want to try out first - you can see this take shape in the CI branch.

I think it is brilliant that you got this working. Before when I looked at AppVeyor, it was not possible to complete the build within the 60 minutes available. You have proven that the build now only takes about 50 minutes, thank you!

I'll share another update when I make some more progress. Thanks again.

[AndrewSav]

@maxwell-k we also now have github actions which allows for 6 hours builds. This is another option.

Any progress on this one?