AppVeyor for Windows CI

This pull request includes the following changes:
* updated mysql and sqlite
* slight change to *install.neko*: when building *nekoml.std*, refer *nekoml.n* from *../boot/nekoml.n*
* slight change to *release.neko*: 7zip's command line version is `7za` instead of `7z`

The current build result of my fork can be seen at https://ci.appveyor.com/project/andyli/neko
AppVeyor is able to store the built zip file, which is pretty cool. It can be downloaded from the *ARTIFACTS* tab.