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