Gas Optimizations

[code423n4]

1. no need to initialize variables with default values Gravity.sol#L54 Gravity.sol#L128 Gravity.sol#L231 Gravity.sol#L233 Gravity.sol#L263 Gravity.sol#L453 Gravity.sol#L568 Gravity.sol#L579 Gravity.sol#L659 Gravity.sol#L660

2. use ++i instead of i++, i+=1 Gravity.sol#L128 Gravity.sol#L233 Gravity.sol#L263 Gravity.sol#L453 Gravity.sol#L568 Gravity.sol#L579 Gravity.sol#L660