Added dependency comparison type /relation in Debian notation.

This commit added an additional dependency comparison type / relation in Debian
notation in contrast to the existing one in mathematical notation.
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.
apt/package.py: Fixed docstring of BaseDependency.version property.