In order to cleanup correctly the extracted source package, the Source
object must be carried by the Changes. Previously two instantiation of
the Source object was made and only one cleaned-up, leaving dandling
files in the importer directory.

Rework accept email subject to avoid confusion over an actual accepted mail from the official archive (Closes: #119)

This was initially implemented with testing in mind. We can now use
self.settings in unittests. Therefor is it no longer required.

Signed-off-by: Mattia Rizzolo <mattia@debian.org>

* OfficialPackage() now takes a dsc upon creation
* OfficialPackage() checks files based on given dsc
* Dsc() have name, version, orig and orig_asc properties available
* Adjust importer, getorigtarball and tests