Replaced pre_depend with rawtype in BaseDependency.__repr__.
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.
Fixed docstring of BaseDependency.version property.
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.