Packaging and other changes to prep for Debian upload

Hi Marius!

Here are a bunch of changes I think are necessary for uploading to Debian.  As you can see, I switched the package to using a Python 3 shebang line, and dh_python3.  I modernized the d/rules file to keep it more in line with the LibraryStyleGuide.  I rewrote the manpages to use reST and rst2man to generate them (reST manpages rule! :).  I also added a very minimal manpage for gtimelogrc, but that could use some fleshing out (I couldn't find much in the published documentation).  I updated the debian/watch file.

A few things I didn't do, but probably should be done at some point:

* Add a manpage for timelog.txt format?
* Prune out the ppa and ubuntu changelog entries?
* Clean up and/or override the lintian warnings:

% lintian *.deb
W: gtimelog: copyright-refers-to-versionless-license-file usr/share/common-licenses/GPL
W: gtimelog: copyright-without-copyright-notice
W: gtimelog: image-file-in-usr-lib usr/lib/python3/dist-packages/gtimelog/gtimelog-large.png
W: gtimelog: image-file-in-usr-lib usr/lib/python3/dist-packages/gtimelog/gtimelog-small-bright.png
W: gtimelog: image-file-in-usr-lib usr/lib/python3/dist-packages/gtimelog/gtimelog-small.png
W: gtimelog: image-file-in-usr-lib usr/lib/python3/dist-packages/gtimelog/gtimelog.png

If you like these changes and pull them into gtimelog:master, also let me know which of the above changes you'd like me to do too.  I think we're nearly ready for uploading to Debian, after which I will sync into Ubuntu.

FWIW, after building the package locally and testing on a Debian sid machine, gtimelog 0.9.0 seems to work well!