minePendingTransactions(miningRewardAddress) {
    const rewardTx = new Transaction(
      null,
      miningRewardAddress,
      this.miningReward
    );
    this.pendingTransactions.push(rewardTx);

    const block = new Block(
      Date.now(),
      this.pendingTransactions,
      this.getLatestBlock().hash
    );
    block.mineBlock(this.difficulty);

    debug('Block successfully mined!');
    this.chain.push(block);

    this.pendingTransactions = [];
  }
Takes all the pending transactions, puts them in a Block and starts the
mining process. It also adds a transaction to send the mining reward to
the given address.

@param {string} miningRewardAddress