Make an abstraction layer for databases

[citorva]

Sielo uses SQLite as a database system. In order to make sielo compatible with last versions and avoid any problem due to database modification, a layer might be implemented. It needs:

• Check if a table exists and create it else
• Check if the table has correct field. Otherwise creates missing fields and attempt to fix bad fields.
• Make an interface between core and database (SQL request must pass through this interface in order to send request)

In order to be flexible with a database, the interface needs to be a trait and implement for each supported database system (SQLite...)

[citorva]

The pure-implementation is done. It is now necessary to write tests in order to make this code safe and clean up code. It is also necessary to write documentation