EJSON support

So, I took another crack at adding EJSON support. I didn't enable it by default to avoid breaking anything that managed to work without it, but if you add `{ use_ddp: true }` when initializing the DDP client, it will use it. I figured when 0.4.0 comes out we can enable by default.

I also added a couple basic tests. @sarfata or @tmeasday you wanna check this out first?