Add `prepend` option to BrowserifyAsset and update readme

### Synopsis:
Adds an optional `prepend` option to `BrowserifyAsset`. This will allow the Browserify asset to include another javascript asset at the beginning of the file it generates. 

### Use Case:
A specific use case is discussed in issue #82 and in conjunction with PR #107 should allow for a single asset to serve a bundle of all of a client apps JS files **and** it's templates via a single script tag.

### Example:
_The example project described below can be found [here](https://github.com/bryanchriswhite/AssetRackExample)_

Project directory structure:
```
AssetRackExample
├── app.js
├── assets.js
├── public
│   ├── index.html
└── widget
    ├── app
    │   ├── controllers
    │   │   └── index.js
    │   ├── directives
    │   ├── index.js
    │   └── views
    │       ├── view1.html
    │       └── view2.html
    └── test
        ├── e2e
        └── spec
```

#### Server
In assets.js (/assets.js):
```javascript
var rack = require('asset-rack')
  , assets = new rack.Rack([
    new rack.BrowserifyAsset({
      url     : '/js/widget.bundle.js',
      filename: __dirname + '/widget/app/index.js',
      prepend : new rack.AngularTemplatesAsset({
        url    : '/js/widget/templates.js',
        dirname: __dirname + '/widget/app/views'
      })
    })
  ])
  ;

module.exports = assets;
```

In app.js (/app.js):
```javascript
var assets = require('./assets');
app.use(assets);
```

#### Client
In index.html (public/index.html):
```html
<script src='/js/widget.bundle.js' type="text/javascript"></script>
```

In index.js (widget/app/index.js):
```javascript
angular.module('myApp', [])
  .config([
    '$routeProvider', function($routeProvider) {
      $routeProvider
        .when('/one', {
          templateUrl: 'view1.html',
          controller : 'MainCtrl'
        })
        .when('/two', {
          templateUrl: 'view2.html',
          controller : 'MainCtrl'
        })
    }
  ])
  .run(['$templateCache', angularTemplates]);
require('./controllers');
```

Should wrap up issue #82