Add method to create debian packages.

This pull request adds a method to create development debian packages.
The packages are named 0.0.0-dev<number of git revisions> . This ensures strictly descending version numbers that are independent of the current date and therefore e.g. allow multiple builds per day.

There is *scripts/create-debian-changelog.py* which creates a proper changelog from all commit messages in the history, but a command to quickly create an empty changelog is also documented.

Tested on Ubuntu 12.04.

Multiple aspects of this might need to be improved in the future - I'm still learning how to create debian packages in a proper way.

Please feel free to tell me your feedback or propose ways to improve this!

I'm OK with this pull request to be licensensed under any license Mailpile itself is licensed under.