Add Snoovatars.

Snoovatars feature for gold user.  Design/concept and prototype by @powerlanguage, backend by @spladug.  Currently looks like this:

![screen shot 2014-10-20 at 5 00 11 pm](https://cloud.githubusercontent.com/assets/2260961/4711136/4e204472-58b5-11e4-996e-b43cbf050dd5.png)

This feature allows gold users to create a custom "snoovatar".  Users can choose to make their creations publicly available via the `/user/<username>/snoo` URL.

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## frontend
Since the js is pretty one-off, I added way more comments than we normally do.  `snoovatars.js` is structured thusly:

1. **init code** – top half of the file, all of the init code is done via [jQuery Promises](http://api.jquery.com/jQuery.Deferred/).  The general flow is:
    * `tailors.json` and existing snoovatar data are passed in from the template
    * spritesheet images defined in `tailors.json` are preloaded
    * when the images are ready, `Tailor` objects are created to manage each configurable part of the snoovatar
    * a `Haberdashery` object is instantiated to manage the tailors and overall state of the snoovatar widget
    * canvas elements and dynamic UI elements are generated and added to the DOM,
    * a generic `view` object is created and event handlers are bound to hook up UI elements to methods of the `Haberdashery`

2. **"classes"** — bottom half of the file, defines a couple of one-off object types used to manage the widget:
    * `CanvasArray` contains shared code between `Tailor` and `Haberdashery` types (basically, having a canvas property and an array of things).
    * `Tailor` manages a single category of things that users can customize about their snoovatar (e.g. hats, glasses, etc.).
    * `Haberdashery` manages state, handles importing and exporting data, and renders the composite snoovatar from the configured `Tailor` objects.

## backend
the configured snoovatar is stored as a blob of json, so the backend is pretty simple.  @spladug, the only thing I changes (i think) is that I ended up wanting to store `key:value` pairs rather than just an array of `value`s, so I swapped out the `VPrintable` validator with `VJSON`, which seems to work fine.

## design/art assets
Design, spritesheet images, and data in `tailors.json` is likely to change, as @powerlanguage is still working on some of those assets and getting feedback from the design mailing list.  Feel free to comment, though.  I tried to keep the structure of `tailors.json` flexible, and derive as much of the UI from it as possible to minimize the need to edit the code when making simple updates in the future (e.g. adding new graphics).

:eyeglasses: @powerlanguage @spladug @Deimos
:no_bell: De-nest and genericize here and everything below (`.arrow-left` instead of `left` to reduce conflicts and increase reusability, etc.)