Enhancements for contact fields, retrieve/update/create, batches and group create

Hi @aliang, in the past couple months I've done a fair number of changes to the gem as I needed functionality for our Google contacts sync feature. I also tried to merge in your changes since my last pull request. I've documented the main changes in the updated README.markdown.

I figured it would be great to suggest these features be added back into the base gem to allow others to more easily use them.