Implement blackbox in Golang (#250)

* Initial release

models/crypters.go

@@ -0,0 +1,15 @@
+package models
+
+// Crypter is gpg binaries, go-opengpg, etc.
+type Crypter interface {
+	// Name returns the plug-in's canonical name.
+	Name() string
+	// Decrypt name+".gpg", possibly overwriting name.
+	Decrypt(filename string, umask int, overwrite bool) error
+	// Encrypt name, overwriting name+".gpg"
+	Encrypt(filename string, umask int, receivers []string) (string, error)
+	// Cat outputs a file, unencrypting if needed.
+	Cat(filename string) ([]byte, error)
+	// AddNewKey extracts keyname from sourcedir's GnuPG chain to destdir keychain.
+	AddNewKey(keyname, repobasename, sourcedir, destdir string) ([]string, error)
+}