WIP: Short URL resource for images

This PR introduces a new resource: `Imbo\Resource\ShortUrl`

The resource will generate short URLs for image resources (with or without transformations). The short URLs will be generated on demand, and each request against an image URL will have an `X-Imbo-ShortUrl` response header which includes the short URL.

When an image is deleted, all short URLs stored for that public key/image identifier combination will be removed.

The format of the short URL is as follows:

`http://<host>/s/<id>`

where `<id>` matches the following regular expression:

`[a-zA-Z0-9]{7}`

Some features that might be added before we merge this branch:

* Add configuration options for pre-generating short URLs for a set of transformations upon adding new images

Thoughts?