Make parse_dsc a classmethod of DscFile

so we have the object creation close to the object itself.
Use Popen.communicate instead of subprocess call

so we can capture stderr and use it in error messages intead of spewing
it on the console.
Silence dpkg --compare-versions

and add the error detail to the exception message. This silences the
testsuite and makes sure we have error message and exception output in
sync.