Adding man page for gnome-pomodoro binary

Hi,

In Debian every single binary must have its own manpage, so I wrote one very short for the gnome-pomodoro binary.
Tell me if I have some things to change and if you're ok with that.

I've written that for the 3.12 branch but adapted it for master. If it fits you for master I can also make a pull request for gnome-shell 3.12 (slight changes in makefile.am as they differ).

Thanks in advance,
Joseph