AppVeyor builds all branches per default.
Add a configuration setting to only build the master branch.

See http://www.appveyor.com/docs/branches#white-and-blacklisting for details.

For the record, AppVeyor builds can skipped by annotating the commit message
according to http://www.appveyor.com/docs/how-to/skip-build
Useful for e.g. website commits.

[skip appveyor]