Handle ETH as token with 0x0 address

[paulperegud]

1) change tx structure to include address of token 2) treat ETH as token with zero address 3) to prevent malicious tokens from preventing root chain from finalizing exits, have separate priority queue for each of the tokens. This way malicious token can only stop its own operations

[paulperegud]

I think this one is ready to merge.

[kasima]

@kfichter – we'll need to cherry pick this merge into the plasma-contracts repo.