Add 'debugOnly: true' to package declaration

Added `debugOnly` declaration on `package.js` to prevent the package from even existing on production builds, like [Velocity does](https://github.com/meteor-velocity/velocity/blob/master/package.js#L9).

I know the package is currently only supposed to work if URL is `localhost`, but I'm not sure that would be an effective protection for data on production, or at least it will prevent some users to feel comfortable enough to use the package.