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 each tile from the server, and composites the
   results into a photomosaic of the original image.
3. The composited photomosaic should be displayed according to the following
   constraints:
    - 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|css)/*    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;
 - use HTML5 features where appropriate;
 - allocate about 3 hours to do the task.
 - make your first submission final. Subsequent changes may not be considered.

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.

Your submission must NOT:
 - use 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 any build tools other than npm (e.g. Browserify, webpack, etc)
 - resize or scale the uploaded image
 - modify the server

Have fun!