Added the ability to import a changelog and tests.

* Added test option to Makefile. Now you can run 'make test'.
* Created lib/debian_package_test.py, which tests lib/debian_package.py.
* Created lib/test_files, for storing files used in tests. Added files:
  - test_changelog: A valid changelog containing all of the complexities
    for which we're testing.
  - test_changelog_invalid_keyword
  - test_changelog_invalid_urgency
* Modified protobufs that I had defined to better suit the current
  implementation. These are still DEFINITELY NOT STABLE and should
  be expected to make incompatible changes at any time.