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

> "Meta-metadata is specifically filtered from transaction submission response." - This doesn't appear to be true currently.

It is true, currently. `tx.submit(function(err, res) { /* res.mmeta does not exist */ })`

This is where we associate a Meta class with an incoming transaction https://github.com/ripple/ripple-lib/blob/develop/src/js/ripple/remote.js#L719-L723

This is where it's filtered from transaction submission result https://github.com/ripple/ripple-lib/blob/develop/src/js/ripple/transactionmanager.js#L120-L122 (for similar reasons that you'd not like to return Amount objects in the new balancechange thing)

Again, you should just extend the Meta class and `tx.mmeta` will be available wherever it should be. 

>  The fixtures were generated from the transaction submission response and contain all the metadata.

You're thinking of transaction metadata, not this meta-metadata (Meta class) that we associate with a transaction when it's streamed from rippled.