[TASK] Add balance_change array to transaction response, computed from metadata

Well I'm not sure if it is absolutely necessary to JSON stringify it, but
1. Right now it looks nice and clean when you JSON stringify it
2. The CLI will be dumping JSON responses in verbose mode, which is very useful for debugging, making test cases etc
3. There's a very high chance that developers will want to use the JSON of it at some point
4. Developers (and ourselves) are definitely going to be looking at JSON representation during development

I'm thinking from the standpoint of a UI developer in the browser. They just want to get all the balance changes so they can show a transaction summary to the user. They probably don't want to be tied to our internal object structures, they just want the information in a simple consistent form.