Remove yepnope and load the resources ourselves

This paves the way for separating the method of script injection from the initialisation code. In future this will allow the Chrome and FF extensions to provide custom code to inject the app into the isolated extension context.

This method now exports a `hypothesisInstall` function that can be provided with custom methods for injecting scripts and stylesheets. e.g. for the Chrome Extension we would use:

```js
 window.hypothesisInstall({
  script: function (src, fn) {
    chrome.tabs.executeScript(tab.id, {file: src}, fn);
  },
  stylesheet: function (href, fn) {
    chrome.tabs.insertCSS(tab.id, {file: href}, fn);
  }
});
```

I've issued this as a PR in a step towards getting the Chrome Extension loading all scripts using `chrome.tabs.executeScript`. The next hurdle here is that the extensions cannot use `executeScript` to load assets over `http` so we need to copy them into the bundle for local development and use relative uris.