Sigh, dsc messages may not include their own checksums (for obvious
tail-eating reasons) but said checksums are needed for including in
apt source repos, so calculate them on the fly.
Also rename the `files` property to `source_files` so that it does
not collide with the Files: header in the dsc message.
Update README appropriately
- python3 compatibility
- drop py2.6 support
- use email.message rather than rfc822.message
- add some initial debug logging
- pylint and pep8 fixes
- add object properties for file hashes
- add a simple cli demo script
- add travis for continuous build