convert entity classes to Kotlin data classes (#526)

* convert entity classes to Kotlin data classes

* more data classes, code style

app/src/main/java/com/keylesspalace/tusky/entity/AppCredentials.kt

@@ -0,0 +1,23 @@
+/* Copyright 2017 Andrew Dawson
+ *
+ * This file is a part of Tusky.
+ *
+ * This program is free software; you can redistribute it and/or modify it under the terms of the
+ * GNU General Public License as published by the Free Software Foundation; either version 3 of the
+ * License, or (at your option) any later version.
+ *
+ * Tusky is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even
+ * the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General
+ * Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along with Tusky; if not,
+ * see <http://www.gnu.org/licenses>. */
+
+package com.keylesspalace.tusky.entity
+
+import com.google.gson.annotations.SerializedName
+
+data class AppCredentials(
+        @SerializedName("client_id") val clientId: String,
+        @SerializedName("client_secret") val clientSecret: String
+)