additional check for invalid share & indentation

[mining-visualizer]

Hey, this is my first PR ever!

I messed up your indentation awhile back and its been bugging me, plus I wanted to add in that extra check for claimedTarget:

          var digestBigNumber = web3utils.toBN(digest);
          var claimedTarget = self.getTargetFromDifficulty( difficulty )

          if(computed_digest !== digest || digestBigNumber.gte(claimedTarget))
          {
            validJSONSubmit = false;
          }

I managed to get the pool software working on my Ubuntu laptop so I had a chance to test it out and it works ok.

[mining-visualizer]

I just submitted a new pull request with the modified hash rate calculations, and it contains the changes in this one as well, so closing this one.