Refactor internal coin representation from BigInteger to long.
Move coin related methods and constants from Utils into Coin.
Clean up Coin API for multiplications and divisions.
Remove weird constructors.
Remove weird value accessors.
Rename Coin.toNanoCoin(String) to Coin.parseCoin(String).
Rename Coin.toNanoCoins(coins, cents) to Coin.valueOf(coins, cents).
Make more use of Coin constants.