Add work with signed `.dsc` files

[LeonidVas]

Now, if we use a signed .dsc file, we get an error like:

Traceback (most recent call last):
  File "/Workspace/mkrepo/./mkrepo", line 5, in <module>
    mkrepo.main()
  File "/Workspace/mkrepo/mkrepo.py", line 117, in main
    update_repo(path, args)
  File "/Workspace/mkrepo/mkrepo.py", line 56, in update_repo
    debrepo.update_repo(stor, args.sign, args.temp_dir, args.force)
  File "/Workspace/mkrepo/debrepo.py", line 851, in update_repo
    process_index_units(repo_info, tempdir, 'sources')
  File "/Workspace/mkrepo/debrepo.py", line 707, in process_index_units
    unit.parse_dsc(local_file, file_path, mtime)
  File "/Workspace/mkrepo/debrepo.py", line 194, in parse_dsc
    self.parse_string(file.read())
  File "/Workspace/mkrepo/debrepo.py", line 235, in parse_string
    key, value = line.split(':', 1)
ValueError: not enough values to unpack (expected 2, got 1)

Example of the signed .dsc file: https://ftp.debian.org/debian/pool/main/3/389-admin/389-admin_1.1.35-2.dsc Documentation: https://debian-handbook.info/browse/da-DK/stable/sect.source-package-structure.html

[NickVolynkin]

@ylobankov this is resolved in #61, isn't it?