Write response back to the client

Not writing anything is bad, just tell the client that everything went
ok when joining/leaving.
Code cleanup.

Remove useless imports
Delete png files after compilation succeeded

If compilation of a paper succeeded, generated png pages are now
probably obsolete.
Tell client that the response is gzipped

As requested in #20, to make the browser gunzip the content itself,
setting the right header should do the trick.
I am not sure whether it is the best solution however, how do other HTTP
clients react to this? We should investigate further if this is
portable.