Support client caching

I'm trying to serve assets using this crate, but I've noticed the server does not send an Etag. This would cause the client to request the asset every time it's referenced (so, on every page load).

The express framework is sending an etag on static mounts, the client would memorize this and include it in future requests, if the server notices the file hasn't changed, it's sending back an 304.

This is critical for page speed and bandwidth. :)