Add gpg support and custom (sub)process module

securesystemslib/gpg/__init__.py

@@ -0,0 +1,23 @@
+"""
+<Module Name>
+  gpg
+
+<Author>
+  Santiago Torres-Arias <santiago@nyu.edu>
+
+<Started>
+  Nov 15, 2017
+
+<Copyright>
+  See LICENSE for licensing information.
+
+<Purpose>
+  This module was written due to the lack of other python (such as pygpg)
+  modules that can provide an abstraction to the RFC4480 encoded messages from
+  GPG. The closest candidate we could find was the python bindings for gpgme,
+  we oped to use a Popen-based python-only construction given that gpgme is
+  often shipped separately and other popular tools using gpg (e.g., git) don't
+  use these bindings either. This is because users willing to use gpg signing
+  are almost guaranteed to have gpg installed, yet the same assumption can't be
+  made for the gpgme python bindings.
+"""