Add a requirements.txt

pip install -r requirements.txt
Refactor log messages.
Add missing format string in log message.
Delete confusing log message.

The log says that exitmap exited even though modules might (and probably are)
still running.
Add a statistics class.

It's used throughout exitmap to keep track of scanning statistics.  Before the
program terminates, the statistics are shown.
Correctly terminate and print statistics.

We are able to tell when all circuits are finished by keeping track of all
successful circuits and the "CLOSED" stream events.  Finally, we will print
some scan statistics.
Added prerequisites and some headings

exitmap needs some libraries in order to run. I added the names of the
packages and a short information on how to install them under Debian
GNU/Linux. Furthermore I inserted some headings, so it should be
easier to read.