Database: allow mysql, config in ini file

Also check for ini file existence at startup

fioul.ini.example

@@ -1,3 +1,16 @@
+[db]
+backend = sqlite
+
+[sqlite]
+filename = fioul.db
+
+[mysql]
+database = fioul
+user = please change me
+password = please change me
+host = localhost
+port = 3306
+
 [server]
 host = localhost
 port = 9980