Add current alpha of protobuf 3

pkgs/development/libraries/protobuf/3.0.nix

@@ -0,0 +1,12 @@
+{ callPackage, fetchFromGitHub, ... } @ args:
+
+callPackage ./generic.nix (args // rec {
+  version = "3.0.0-alpha-3.1";
+  # make sure you test also -A pythonPackages.protobuf
+  src = fetchFromGitHub {
+    owner = "google";
+    repo = "protobuf";
+    rev = "v${version}";
+    sha256 = "0vzw20ymjmjrrmg84f822qslclsb2q0wf0qdj2da198gmkkbrw45";
+  };
+})

pkgs/top-level/all-packages.nix

@@ -7847,6 +7847,7 @@ let
   postgis = callPackage ../development/libraries/postgis { };
 
   protobuf = protobuf2_6;
+  protobuf3_0 = lowPrio (callPackage ../development/libraries/protobuf/3.0.nix { });
   protobuf2_6 = callPackage ../development/libraries/protobuf/2.6.nix { };
   protobuf2_5 = callPackage ../development/libraries/protobuf/2.5.nix { };