Sqlite as storage backend (#1)

Some platforms has some problems with file locking, so I was forced to use an alternative. SQLite seems be the best option currently available


* Migrated to an sqlite database

* Removed unnecessary IOExceptions

* Removed an util class not needed anymore

* Updated README.md and docker-compose.yml to reflect new storage mechanism

.gitignore

@@ -10,3 +10,4 @@ url.iml
 .settings/
 urls.csv
 .env
+urls.sqlite