disable Travis success notifications

Email notifications would only trigger when the status changes from broken to
good, but rust-ci triggers a rebuild every day, so when you do not commit, it
will keep sending a 'build fixed' email every day when the latest commit is one
that fixed a failing build. That is too verbose for me.
bump dependencies
do not explicitly allow if_let

The feature has been added to the language, a directive is no longer necessary.
bump dependencies
derive Copy where necessary

Copy is not derived by default any more, so it needs to be added explicitly.
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