Update package.json

* I updated `repository` field of package.json because [azu/remove-strict-mode](https://github.com/azu/remove-strict-mode) doesn't exist.
* I updated [esprima](https://www.npmjs.org/package/esprima), [power-assert](https://www.npmjs.org/package/power-assert), [mocha](https://www.npmjs.org/package/mocha) and [espower-loader](https://www.npmjs.org/package/espower-loader) to the latest version.
* I changed the image format of build status badge from PNG to SVG. SVG badges look beautiful on retina displays.
* I added `.gitignore` file.