replace proc with move ||
add command-line options to run in master or slave mode
replace fail! with panic!
allow specifying the port in master mode
add sender to send render results over network
implement the send task
print send task result
implement merging received images
implement pop_iter again

I just removed it from master, but it _was_ used in this branch, not
with a `RingBuf`, but with a `Vec`. So now this has a specialised
`pop_iter` for `Vec`. Again, I liked the generic one better, but this
suffices for now.
update TcpStream::connect
add listener
implement listening in master mode
derive Copy for AppMode
replace proc with move ||