Add package.json

Thank you for providing support for using Dexie as a CommonJS module. This adds a package.json file to the root so that Dexie can be installed with npm.

I made the version 0.9.8 instead of 0.9.8.1 because node only allows [semvar](https://www.npmjs.org/doc/package.json.html#version) version numbers in the format `x.y.z`.