Proposal: implement a method in WS API to get address balances

[bonustrack]

In the current WS API there is no easy way to get an account balances. There is the method light/get_history which give in and out transactions of a specific address(es) but it's not that easy to compute the balances giving the possibilities that there could be smart contract, asset created or it could simply not work if there is more transactions than the MAX_HISTORY_ITEMS limit. It would be great to have directly the balance from a new API method without having to compute it yourself from history.