Plasma Transaction structure + merkle proofs + struct size detect

Structure:
```solidity
struct Transaction {
    Input[] inputs,
    Output[] outputs,
    uint64 maxBlockIndex,
    Signatures[] signatures
}
```

Merkelized:
<img width="425" alt="image" src="https://user-images.githubusercontent.com/702124/48621082-753ab680-e9b3-11e8-8de7-ae3d44046dd1.png">