Remove json dependency

@danielbreves @seancaffery @svizzari

This project already had MultiJson. There was no reason to tie it specifically to the json gem, which requires C extensions. MultiJson is more portable and will use whichever JSON codec is available.

The one downside is having to replace calls to `some_variable.to_json` with `MultiJson.encode(some_varible)`.

An added benefit is that you don't need Xcode to install ZAS (and therefore ZAT) on OSX :)