add (disabled) Signal

With Whisper Systems making Signal work without Google Play Services and
Copperhead's Google-free fork, Signal is finally coming quite close to
being ready for inclusion in F-Droid.  I'm adding it is here to make it
easy to work together on making it build reproducibly.

metadata/org.thoughtcrime.securesms.txt

@@ -0,0 +1,55 @@
+Categories:Security,Phone & SMS
+License:GPL-3.0
+Web Site:https://signal.org
+Source Code:https://github.com/WhisperSystems/Signal-Android
+Issue Tracker:http://support.whispersystems.org
+
+Auto Name:Signal
+Summary:Privacy is possible, Signal makes it easy.
+Description:
+ Using Signal, you can communicate instantly while avoiding SMS fees,  create
+groups so that you can chat in real time with all your friends  at once, and
+share media or attachments all with complete  privacy. The server never has
+access to any of your communication and  never stores any of your data.
+
+ ★ Say Anything. Signal uses an advanced end to end encryption  protocol that
+provides privacy for every message every time.
+
+ ★ Open Source. Signal is Free and Open Source, enabling anyone to  verify its
+security by auditing the code. Signal is the only private  messenger that uses
+open source peer-reviewed cryptographic protocols  to keep your messages safe.
+
+ ★ Be Yourself - Signal uses your existing phone number and address  book. There
+are no separate logins, usernames, passwords, or PINs to  manage or lose.
+
+ ★ Group Chat. Signal allows you to create encrypted groups so you can  have
+private conversations with all your friends at once. Not only  are the messages
+encrypted, but the Signal server never has access to  any group metadata such as
+the membership list, group title, or group  icon.
+
+ ★ Fast. The Signal protocol is designed to operate in the most  constrained
+environment possible. Using Signal, messages are  instantly delivered to
+friends.
+
+ ★ Speak Freely - Make crystal-clear phone calls to people who live  across
+town, or across the ocean, with no long-distance charges.
+
+ Signal is not currently compatible with tablets, but support for  larger
+screens is on our roadmap and will be included in a future  release!
+
+ For support, questions, or more information, please visit:
+.
+
+Repo Type:git
+Repo:https://github.com/WhisperSystems/Signal-Android
+
+Build:4.2.3,256
+    disable=includes proprietary Google Play Services GMS libs
+    commit=v4.2.3
+    gradle=yes
+    rm=libs/*/*.so
+    ndk=r13b
+
+Archive Policy:0 versions
+Auto Update Mode:None
+Update Check Mode:None