Rewrote JavaScript to RequireJS

Why AMD/RequireJS?
See http://requirejs.org/docs/why.html

**What's done**:
* Added requirejs-rails gem (from git ref, since 0.9 release has uglify bug)
* Moved and changed existing modules to use AMD syntax
* Replaced most inline javascript with widgets, that are initialized by adding a data-widget attribute to html-elements you wish to enhance with javascript
  * Except where dynamic data is passed from the server (see /votes/xxx)
* Created rake js:install to install npm dependencies (not sure how to re-implement retry 2a88e6e)
* Changed Buster to use requirejs with the help of buster-amd

**Todo**:
* Asset pipeline doesn't work for buster tests, so probably have to write a plugin to support this. Something like this? https://github.com/jwhitley/requirejs-rails-jasmine-template
* If html content loaded through ajax needs to be javascript-enhanced, we need to re-run the code in application.js without doing it again for everything already initialized. Haven't found any existing features that needs this yet. So not implemented.


**Technical details** (also in application.js):

JavaScript should now be defined using AMD-syntax:
http://addyosmani.com/writing-modular-js/

RequireJS docs: http://requirejs.org/docs/api.html

Without AMD:
```
var myModule = (function ($) {
  return {...};
})(jQuery);
```

With AMD the file-name is the name of the module, so define it anonymously:
myModule.js:
```
define(["jQuery"], function ($) {
  return {...};
});
```

You can depend on scripts using the assets-pipeline name, and add a shim config in requirejs.yml if it is not using AMD. jQuery-plugins typically don't have to use exports. See requirejs docs for more info about shims: http://requirejs.org/docs/api.html#config-shim
```
shim:
  "highcharts":
    deps:
      - "jquery"
    exports:  "Highcharts"
```

This is the main script for requirejs on every page of the site. Inline script has been replaced with re-usable widgets. In development mode the widgets and dependencies will be loaded async on demand, but for production the new widgets have to be added to bundle.js manually.

A widget is an HTML-element that we want to enrich with JavaScript functionality. Instead of having an inline script initializing it, add a data-widget attribute with the widget module as value. (widget/ is prefixed to enforce all widgets is placed there)

```
<div data-widget="myWidget"></div>
```

application.js will look for any elements with a data-widget attribute and require the module. For the example above, it will load app/assets/javascript/widgets/myWidget.js

Since a widget should be re-usable, it should not hard-code what elements to attach to. Instead it should return a function that takes an element as first argument, and initialize the widget on this element. Ideally a widget is just a thin layer to bind other modules to HTML-elements.

```
define(["jquery", "hdo/myModule"], function ($, myModule) {
  return function (element) {
    myModule.init(element);
  };
});
```

Sometimes you need to pass dynamic data to the widgets. Preferably the widget/module should load this through ajax instead, but it can also use a named define (bad practise):
```
define("myData", function () {
  <%= @myData.to_json.html_safe %>;
});
```

The widget/module have to require the data on demand to avoid compilation errors when building the production bundle.

```
var myData = require("myData"); // notice no array => sync require instead of async
```

What do others think of this? Any feedback greatly appreciated :)