Implement JSON-RPC based deposit contract

[mkalinin]

To Do

Implement integration with Deposit contract that is a Mainnet contract for Beacon chain registration deposits.

Proposal

Built one of implementations on top of Web3j. Hence, any Mainnet client that supports JSON-RPC could be used to fetch contract data.

Spec

https://github.com/ethereum/eth2.0-specs/blob/master/specs/core/0_beacon-chain.md#ethereum-10-deposit-contract

[Nashatyrev]

EthereumJ powered DepositContract was implemented with PR #43

[zilm13]

should be closed with #170