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