Enhanced import/export support.

1) Allow imports from the stock SMS database at any time.

2) Provide plaintext export support, in a format compatible with
   the "SMS Backup And Restore" app.

3) Fix the DB weirdness on encrypted restore that previously
   required killing the app.

src/org/thoughtcrime/securesms/database/SmsMigrator.java

@@ -191,8 +191,8 @@ public class SmsMigrator {
                                      MasterSecret masterSecret,
                                      SmsMigrationProgressListener listener)
   {
-    if (context.getSharedPreferences("SecureSMS", Context.MODE_PRIVATE).getBoolean("migrated", false))
-      return;
+//    if (context.getSharedPreferences("SecureSMS", Context.MODE_PRIVATE).getBoolean("migrated", false))
+//      return;
 
     ThreadDatabase threadDatabase = DatabaseFactory.getThreadDatabase(context);
     Cursor cursor                 = null;