Set cache-control and expires headers to improve browser caching.

I'm assuming most assets deployed with Middleman will use an asset hash. So setting a far-future expires header should be safe.

.html files are not cached. It's up to the browser to work out how to handle html files. Chrome and Firefox seem to be smart about handling .html pages without cache headers. They pull the page from cache for a few minutes and then check back with the server after that.
Merge pull request #4 from willkoehler/add_cache_headers

Set cache-control and expires headers to improve browser caching.