python/cache.cc, apt/package.py: Introduced dependency comparison type / relation in Debian notation.
apt/package.py: Added missing versionadded for relation_deb.
apt/package.py: Replaced pre_depend with rawtype in BaseDependency.__repr__.
apt/package.py: Added rawtype property to Dependency class.

This change also includes changes to the constructor of the Dependency class
and the Version.get_dependencies method to use a common terminology.