Add imgsz option, enrich package.json and readme

1. Added imgsz option - it's quite usefule sometimes, and I've set default value as 'medium' (for details see https://developers.google.com/image-search/v1/jsondevguide#request_format)
2. Added homepage, repository (that's going to ease the pain for people who end up on npmjs.org with just a few examples and no links to source) and incremented version in package.json
3. Added a link to API docs at Google website.

Hope this is helpful and my commits are not in any way offensive to your project.

Thanks!