Firebase emulator testing (#9)

- Firebase emulator added
 - Update to Espresso tests
 - Updated gradle dependencies for espresso
 - Updated config.yml
 - Updated android gradle version

database.rules.json

@@ -0,0 +1,10 @@
+{
+  "rules": {
+    "user": {
+      ".read": true,
+      "$user_id": {
+        ".write": "$user_id === auth.uid"
+      }
+    }
+  }
+}