Check if the specified APT proxy server can be reached, informing the
user if this can't be done. That will help them to debug the errors that
will be raised by `apt-get` because of the misleading proxy
configuration.
This closes#95.
Three of the last developed plugins remained on the old `plugins/`
folder. This probably happened because they didn't existed when the
`documentation` branch were created, so they weren't moved when it was
rebased against `master`.
