fio2gnuplot: Removing .py extension

It's pretty boring having the .py extension to the tool.
Let's remove it !
fio2gnplot: Adding manpage

As requested by users, let's add a manpage to make things easier to
understand.

I'm providing here the raw file used to generate the manpage. It can be
converted to a real manpage by using txt2man.

I'm also providing the manpage file to avoid dependencies at build time,
anyway the makefile target can be easily used to rebuild it if
necessary.