Allow multiple 'sprocketsPath' entries to be configured.

Hi,

My current project requires libraries to be loaded from multiple asset directories.
This change allows the `sprocketsPath` configuration property be configured as a string or an array of paths.

The current configuration is still supported:

```javascript
    sprocketsPath: 'app/assets/javascripts'
```

Or the paths can be set as an array of strings:

```javascript
    sprocketsPath: [
        'app/assets/javascripts',
        'bower_components'
    ]
```