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Photo mosaic

The goal of this task is to implement the following flow in a client-side app.
1. A user selects a local image file.
2. The app loads that image, divides the image into tiles, computes the average
   color of each tile, fetches a tile from the server for that color, and
   composites the results into a photomosaic of the original image.
3. The composited photomosaic should be displayed according to the following
    - tiles should be rendered a complete row at a time (a user should never
      see a row with some completed tiles and some incomplete)
    - the mosaic should be rendered from the top row to the bottom row.
4. The client app should make effective use of parallelism and asynchrony.

The project skeleton contains a lightweight server (written in node) for
serving the client app and the tile images. To start it, run npm start.
  /              serves mosaic.html
  /js/*          serves static resources
  /color/<hex>   serves an SVG mosaic tile for color <hex>.  e.g., /color/0e4daa

The tile size should be configurable via the code constants in js/mosaic.js.
The project skeleton is already set up to include those constants in both the
mosaic client and the mosaic server.  The default size is 16x16.

You should:
 - pretend you're submitting this as production-quality code for review; i.e.,
   - write effective comments; 
   - make the code modular;
   - make the code testable;
 - avoid using JS libraries (e.g., jQuery, Modernizr, React) as browser APIs
   are sufficient for the exercise and we're not testing for familiarity with
   any particular tools;
 - use HTML5 features where appropriate;
 - allocate about 3 hours to do the task.

You may:
 - edit /etc/hosts;
 - use any HTML5 feature supported by current Chrome (e.g., Promise, Worker);
 - be as creative as you like with the submission UI (file input, drag & drop,
   etc); however, it is not the focus of the task, a minimal UI is fine.

Have fun!