Start of port to Python 3

This isn't going to be quick since we need a python3 version of
protobufs, but this starts the process by using all relevant
__future__ imports.
Minor adjustments.
Fixed the test changelog.
Abstract Changelog class and start importing control files.

Added sample control file, taken from hello-debhelper.

Restructuring:
* Renamed some protobuf repeated fields to plural. Specifically:
  - uploader -> uploaders
  - additional_field -> additional_fields

debian_package.py:
* Moved to absolute imports
* Moved functions that perform tasks for changelogs into a new class.
* Added _Package.parse_priority class method to handle priorities.
* Added SourcePackage._parse_control_line for parsing lines from control
* Added SourcePackage.import_control_file for importing control files.
* SourcePackage.import_changelog_file:
  - Can now start anywhere in a file.
  - Uses person.py's parse_person function to parse maintainer lines.

person.py:
* Created file, added parse_person function.

TODO:
* Complete tests. I would have done it tonight but I need to get to bed.
* Parse more source package fields.