Issue title: Results are always the same
 Issue body: I'm creating an application to compare one image to multiples images in a folder. I'm creating this application with Electron and React.

I pass 2 Buffers. As you can see, the Buffers are the same at the first entry, but it gives the same results.
<img width="844" alt="capture d ecran 2017-11-02 a 11 26 40" src="https://user-images.githubusercontent.com/1322006/32334572-25d36b18-bfc1-11e7-9724-20c9be1fe8f2.png">

There's how I use rembrandt:

```javascript
        let mainImg = fs.readFileSync('/Users/mboutin2/Desktop/12798992_10207498962925066_4475804348577634517_n.jpg');

        data.forEach((file) => {
          smb2Client.readFile('Mike\\' + file, function(err, data){
            if(err) {
              console.log("Error (readdir):\n", err);
              console.log("data", data);
            } else {
              //console.log("File ->", file);

              const rembrandt = new Rembrandt({
                // `imageA` and `imageB` can be either Strings (file path on node.js,
                // public url on Browsers) or Buffers
                imageA: mainImg,
                imageB: data,

                // Needs to be one of Rembrandt.THRESHOLD_PERCENT or Rembrandt.THRESHOLD_PIXELS
                thresholdType: Rembrandt.THRESHOLD_PERCENT,

                // The maximum threshold (0...1 for THRESHOLD_PERCENT, pixel count for THRESHOLD_PIXELS
                maxThreshold: 1,

                // Maximum color delta (0...255):
                maxDelta: 20,

                // Maximum surrounding pixel offset
                maxOffset: 0,

                renderComposition: false, // Should Rembrandt render a composition image?
              });

              rembrandt.compare()
                .then(function (result) {
                  console.log('\n\n\n');
                  console.log(data);
                  console.log(mainImg);
                  console.log('Passed:', result.passed)
                  console.log('Pixel Difference:', result.differences, 'Percentage Difference', result.percentageDifference, '%')
                  console.log('Composition image buffer:', result.compositionImage)

                  // Note that `compositionImage` is an Image when Rembrandt.js is run in the browser environment
                })
                .catch((e) => {
                  console.error(e)
                })
            }
          });
        });
```

Is there something I do wrong?
 Comments: 
 Comment 0: Did you get this to work? I do only get that they are not the same image, but they are. Here is my test: http://cdn.axiell.com/~dev/gota/image-test/html-template.html

I use this code:

`const rembrandt = new Rembrandt({
      // imageA and imageB can be either Strings (file path on node.js,
      // public url on Browsers) or Buffers
      imageA: $('.image1').attr('src'),
      imageB: $('.image2').attr('src'),

      thresholdType: Rembrandt.THRESHOLD_PERCENT,

      // The maximum threshold (0...1 for THRESHOLD_PERCENT, pixel count for THRESHOLD_PIXELS
      maxThreshold: 0,

      // Maximum color delta (0...255):
      maxDelta: 0,

      // Maximum surrounding pixel offset
      maxOffset: 0,

    })

    // Run the comparison
    rembrandt.compare()
      .then(function (result) {

        if(result.passed){
          alert('Yes');
        } else {
          alert('No');
        }
  })`
 Comment 1: I seems to have solved this part. I made maxDelta: 20 and it started to set "passed" to correct value.
{'source': 'GithubIssues', 'repo_name_with_owner': 'imgly/rembrandt', 'issue_id': 270699205, 'repo_stars': 271.0}