Start having useful defaults for SQL

Mostly this means: Havintg a config_spec.ini that has a
local (relative to mediagoblin.ini) sqlite db with the name
"mediagoblin.db".

Also:
- Add to .gitignore
- Add a notice to mediagoblin.ini about the db

mediagoblin.ini

@@ -5,6 +5,10 @@
 direct_remote_path = /mgoblin_static/
 email_sender_address = "notice@mediagoblin.example.org"
 
+## Uncomment and change to your DB's appropiate setting.
+## Default is a local sqlite db "mediagoblin.db".
+# sql_engine = postgresql:///gmg
+
 # set to false to enable sending notices
 email_debug_mode = true