add `appveyor.yml` file for Windows CI testing
package: add top-level `npm test` command

For appveyor to use.
appveyor: just invoke the top-level `npm test` command

Apparently using the && syntax in the appveyor file doesn't
work as expected.
Merge pull request #144 from TooTallNate/add/appveyor

add `appveyor.yml` file for Windows CI testing