Preparations for the 1.4 release

PuffinPlot's website has moved again, to http://talvi.net/puffinplot/ . The source code and release hosting has now been moved from BitBucket to GitHub, and the version control system has been changed from Mercurial to Git.

There has been much development since the 1.03 release in 2015, much of it not directly noticeable to the user, but all of it useful. Almost all the data processing code is now covered by an extensive set of unit tests, helping to ensure that PuffinPlot's calculations are correct and that they will remain correct during future improvements to the code.