MD5 cache busting & multiple asset host support

This extends and refactors #60 by @blakeharv. Support for defining an array of asset hosts as a configuration option is added and code is refactored to reduce repetition.

Asset hosts can be configured as such from within the existing options array:

```Ruby
    assets {
      asset_hosts [
        '//cdn-0.example.org',
        '//cdn-1.example.org',
        '//cdn-2.example.org',
        '//cdn-3.example.org'
      ]
      ...
    }
```