Generate "prekeys" at push registration time.

This generates a large number of key exchange messages and
registers them with the server during signup.

protobuf/Makefile

@@ -0,0 +1,3 @@
+
+all:
+	protoc --java_out=../src/ PreKeyEntity.proto

protobuf/PreKeyEntity.proto

@@ -0,0 +1,9 @@
+package textsecure;
+
+option java_package = "org.thoughtcrime.securesms.encoded";
+option java_outer_classname = "PreKeyProtos";
+
+message PreKeyEntity {
+  optional uint64 id  = 1;
+  optional bytes  key = 2;
+}