Generalizing package

* Package renamed to pkg-version.
* Dpkg version scheme is very similar to RPM version scheme and
  therefore it is possible to reuse a lot of code.
* Created PkgVersion data type from RpmVersion, it will be used as base
  type for all derived data types like RpmVersion and DpkgVersion. Most
  importantly they will only differ in Eq and Ord instances.
* Function fromVersion changed to: fromVersion :: [Int] -> PkgVersion
  This way it is general enough to work on anything.