Added package.json to bower's ignore list.

In order to be able to shim this library to use it with [browserify](https://github.com/substack/node-browserify) using [browserify-shim](https://github.com/thlorenz/browserify-shim) we need to at least exclude the package.json from bower package, so browserify can look up the higher level `package.json` for browserify transforms settings.