Rewrite RSpec matchers so that they rely on GnuPG's machine-readable output

[skalee]

Moved from riboseinc/enmail#63.

GnuPG output is intended to be human readable, not to be interpreted by computers. Its format is unstable, so do exit codes.

At riboseinc/enmail#57 I was wrong. GnuPG produces machine readable output when the --status-file <file> option is passed to the GnuPG executable. The format is easy to parse, and keyword synopsis are preserved across versions. Full description is in doc/DETAILS file of GnuPG.

On the other hand, the machine readable output misses some information which is otherwise printed to stderr, or at least it is not easy to obtain. Notably recipient user ids of encrypted message are missing.