Fallback to long id GPG key lookup when fingerprint is not available in signature. Enable gpgv1 signatures to work. (Closes: 120)

GnuPG(): re-order verify_sig statements, fix typo in comment and remove default assignment =[] from get_keys_data signature

Rename GnuPG.add_signature() to GnuPG.import_key(). Remove the capability to import key from file (not used in debexpo)

- Add two new classes:
  - VirtualKeyring, that builds a temporary keyring from a user or a
    standalone key
  - GPGSignedFile, that handle gpg signed files
- Add more exceptions to be precise in the error
- Use --status-fd to get machine-readable data from gpg
- Use GNUPGHOME to create temporary secure keyrings
- Remove some unused methods and classes (GPGAlgo is now in DB)

Currently, gpg keys with multiple uid won't work for the following use
case:

- Package import will fail as the regexp matching gpg uids only match the
primary uid.

- Only the primary uid will be matched using uid fingerprint on user
profile update:

gpg --with-colons < key.gpg fails to print uid fingerprint
gpg --with-colons --import --import-options show-only < key.gpg succeed

https://unix.stackexchange.com/questions/335669/gnupg-2-1-16-with-fingerprint-no-longer-works-to-show-fingerprints

KeyData() and Uid() class imported from nm.debian.org code.
Adding class GPGAlgo() to output old id format present in gnupgv1.

(gpg_path's validity check, 'is_valid' default value)

 - use --status-fd for easier parsing of gpg commands' result
 - add a default gpg path ('/usr/bin/gpg') and keyring ('~/.gnupg/keyring.gpg')
 - verify_file can take data as a string too

(+ add generic exception for missing data in a gpg command)

(Was working only with one-uid keys.)