Add links to AppVeyor status

[techtonik]

To http://aspen.io/ and to GitHub page. This requires API key and I don't know where to get it.

[chadwhitacre]

AppVeyor is supported by http://shields.io/, without requiring an API key:

[![AppVeyor](https://img.shields.io/appveyor/ci/gratipay/aspen-python.svg)](https://ci.appveyor.com/project/gratipay/aspen-python)

[techtonik]

[![AppVeyor](https://img.shields.io/appveyor/ci/gratipay/aspen-python.svg "AppVeyor Build Result") ](https://ci.appveyor.com/project/gratipay/aspen-python)

[techtonik]

On the second look at http://www.appveyor.com/docs/status-badges API key is not required for public repositories, so it is possible to use official badge with a nice logo.

[![AppVeyor](https://ci.appveyor.com/api/projects/status/github/gratipay/aspen-python?branch=master&svg=true "AppVeyor Build Result") ](https://ci.appveyor.com/project/gratipay/aspen-python)

[chadwhitacre]

@techtonik I think we should use the Shields version, because standardization is the reason that Shields exists, and Shields is a Gratipay project. 💃

[pjz]

I did a first pass at this on #482 since this PR kind of depends on it.

[pjz]

Fixed in #482

[techtonik]

@whit537 I am not sure that shields doesn't cache results more than necessary.

[chadwhitacre]

@techtonik Then make an issue/PR over there.