Use webserver

Instead of custom handlers, use the builtin elnode webserver.

# Advantages

We get a lot for free, including

- proper response codes and HTTP headers
- mimetype detection
- caching
- HTML index pages

The first item conveniently fixes #11.

# Disadvantages

We expose the whole `servant/` directory, including the `tmp/` directory.  Hence, the PID file, which is sort of private data, is accessible from the client

In my understanding, servant is not intended for production use, so exposing private server data to the client will do no harm.  Besides, we can simply move the PID file to a different place.

I think, that overall the huge simplification and feature boost justify this little inconvenience.