fix(@embark/transaction-logger): Circular JSON log and unknown contract log level

## Issue
Transaction logs for contracts that were unknown to Embark (ie not in the dapp) would often log super large objects (filling the terminal) that were not formatted with spaces so were hard to read without actually be that useful. In addition, occasionally the object logged would throw the error `TypeError: Converting circular structure to JSON`.

## Fix
Set the log level for transaction logs that are not from a known contract to `debug`, so that they do not flood the terminal with often usused information.

Use `util.inspect` to print the transaction log instead of `JSON.stringify` to prevent circular structure errors.

Author: emizzle

packages/plugins/transaction-logger/src/index.js

@@ -1,6 +1,7 @@
 const async = require('async');
 import { __ } from 'embark-i18n';
 const Web3 = require('web3');
+const util = require('util');
 
 const { blockchain: blockchainConstants } = require('embark-core/constants');
 import { dappPath, hexToNumber } from 'embark-utils';
@@ -169,7 +170,7 @@ export default class TransactionLogger {
 
     const contract = this.addressToContract[address];
     if (!contract) {
-      this.logger.info(`Contract log for unknown contract: ${JSON.stringify(args)}`);
+      this.logger.debug(`Contract log for unknown contract: ${util.inspect(args)}`);
       return this._getContractsList((contractsList) => {
         this.addressToContract = getAddressToContract(contractsList, this.addressToContract);
       });