read static files with filed instead of createReadStream

This tiny change reads static files with `filed` instead of with `fs.createReadStream` so that they are sent with the correct mime-type (currently, an annoying mime-type warning shows up in chrome saying js files were interpreted as js but transferred as text/plain). 

There is a minor trade-off here: because of a [bug](https://github.com/mikeal/filed/issues/27) in `filed`, the stream created by the `filed()` method never actually emits an 'end' event, which means that the `log()` function in beefy doesn't get triggered. IMHO this is better than that annoying mime-type warning. Also, mikeal is bound to fix this sooner or later (hopefully sooner).