Switch to pull payments

[Julian Gruber]

• add state
  mapping(address => uint) balances
• add function
  balanceOf(address account) public view returns uint
  • return state
• add function
  withdraw(address target, uint value) public returns bool
  • support f4/0x addresses
  • support f1 addresses
  • update state (optional gc)
  • transfer funds
  • emit event Withdrawal(address indexed account, address to, uint value)
• add function
  withdrawOnBehalf(address participant, bytes signature, address target, uint value) public
  • verify signature check(signature, from, to, amount)
  • verify balance > 0.1 FIL
  • send 0.1FIL (adjustable) back to sender, to pay for gas
  • withdraw() with different participant address and 0.1FIL deducted
• change function
  reward(address[] addresses, uint[] scores) private
  • update state