Iteratively attempt to sign files
arch_release_promotion/signature.py:
Add sign_file() to sign a single file and return the status code of
the gpg call.
Change sign_files_in_dir() to call sign_file() iteratively per file
until a status code of 0 is emitted.
tests/test_signature.py:
Add tests for sign_file() and adapt tests for the changes to
sign_files_in_dir().
Fixes #6 (closed)