Don't JSON stringify or parse attachments

Ok here's the first chunk of the PR for review.

I've added a test that ensures that binary data that is parseble as JSON will not be parsed, and also updated the fixtures for the other attachment tests.

There are two internal options, dont_stringify and dont_parse that are used by attachment insert and get respectively. 

I'll update the PR with the README changes if you think the approach is good. For the README I was thinking of adding these options to the described options, and describe the changes in the "attachment" section.