Implement ERC20Cogateway::confirmDeposit

[deepesh-kn]

Implement confirmDeposit method in ERC20CoGateway.

• If the utility token for the given ERC20 value token does not exist, then a proxy for utility token contract is deployed.
• Mints the token.
• Merkle proof is required.

   ERC20Cogateway:confirmDeposit
      - value token address
      - amount
      - beneficiary address
      - gas price
      - gas limit
      - sender
      - block number
      - storage proof

• This method should also mint reward/fee for msg.sender

Emit event: DepositIntentConfirmed

DepositIntentConfirmed
     - message hash

Emit event: UtilityTokenCreated

UtilityTokenCreated
    - value token
    - utility token