Remove txBytes parameter from deposit function

[4000D]

With RLPEncode library, we can make plasma transaction in RootChain contract removing txBytes parameter at deposit().

cost type	total gas used
transaction cost	51224 gas
execution cost	29952 gas

[smartcontracts]

We're currently thinking about creating a different type of deposit transaction to avoid having to do this encoding on chain and reduce the gas cost in general. The deposit root would be calculated somewhat like this:

    /**
     * @dev Calculates the block root for a deposit transaction
     * @param depositor Address of the depositor
     * @param amount Amount deposited
     * @return The root to be used for the deposit block
     */
    function calculateDepositRoot(address depositor, uint256 amount)
        private
        view
        returns (bytes32)
    {
        bytes32 root = keccak256(depositor, amount);
        for (uint256 i = 0; i < 16; i++) {
            root = keccak256(root, zeroHashes[i]);
        }

        return root;
    }

This means that an exitor just needs to send the amount when exiting from a deposit.

[smartcontracts]

Closing in favor of #72 which modified deposits so that deposit blocks are represented as keccak256(msg.sender, msg.value).