Implement io.ReadSeeker interface for downloader

Created by: ChrisSchinnerl

This PR adds the /renter/stream endpoint to the renter which is basically another download endpoint that uses the http.ServeContent method to serve a resource. This also allows for opening and streaming pretty much every browser supported file in the browser. (also videos). This is a WIP, contains some nasty caching and disables the Sia-Agent check to make it work in a browser but I would appreciate some overall reviews.