Adding a Query system for databases

[binyamin555]

As of now, we use the SQl Alchemy query() method to create refined filters when accessing the database.

This is exposed through DatabaseRepository which is not so good since this type should be isolated from dependencies.
We should work on a better DB access API which - using the power of inheritance - will allow us to access any DB with EZ defined
query options and types. Of course that it should be extensible so we can add more things easily.

SQL Alchemy will probably still be an EZ dependency and we might only support this library by default.

[neriyaco]

Since we work only with SQL (at least for now) SQL Alchemy is really good with handling all this queries for us,

I don't know if having a different query system is necessary for v1 though. We need to discuss this in more depth.