don't bypass the web of trust (#378) (#379)

* don't bypass the web of trust (#378)

the choice whether to use a web of trust and on which trust level is up
to the user of PGP/GPG and must not be overriden by tools that are set
on top

users can decide to ignore this safety net by setting their gpg.conf
adequately, defining an alias for `gpg --trust-model=always` or passing
the env GPG to blackbox in this way but we should not override their
preferences hardcoded

* update README

add note about the web of trust

* fix broken test

assume that we have `--quick-generate-key` if we run gpg2 instead of
doing a dry run for that (which has side effects that break the test)

pkg/crypters/gnupg/gnupg.go

@@ -106,7 +106,6 @@ func (crypt CrypterHandle) Encrypt(filename string, umask int, receivers []strin
 	a := []string{
 		"--use-agent",
 		"--yes",
-		"--trust-model=always",
 		"--encrypt",
 		"-o", encrypted,
 	}