fix(etherscan): support stringified numbers in response

[mattsse]

Motivation

Closes #1507

the etherscan API returns numbers as strings

    {
      "blockNumber": "2228258",
      "timeStamp": "1473433992",
      "hash": "0x5f2cd76fd3656686e356bc02cc91d8d0726a16936fd08e67ed30467053225a86",
      "nonce": "10",
      "blockHash": "0x466dbced0a98e1b0490f787b51570850c8347960a4e92bd89acf85299e4cb9dc",
      "from": "0x4e83362442b8d1bec281594cea3050c8eb01311c",
      "contractAddress": "0xecf8f87f810ecf450940c9f60066b4a7a501d6a7",
      "to": "0xac75b73394c329376c214663d92156afa864a77f",
      "value": "101000000000000000000",
      "tokenName": "",
      "tokenSymbol": "",
      "tokenDecimal": "1",
      "transactionIndex": "10",
      "gas": "1000000",
      "gasPrice": "20000000000",
      "gasUsed": "93657",
      "cumulativeGasUsed": "684715",
      "input": "deprecated",
      "confirmations": "13002487"
    }

wich results in wrong values

Solution

PR Checklist

• Added Tests
• Added Documentation
• Updated the changelog