Add an Eve-aware base class (WIP)

[nicolaiarocci]

The idea is to provide a base class from which user Tables can inherit. Unless they are meant for read-only acces, tables will have to provide these fields anyway.

Not sure on a few things:

• namespace should be different? Maybe have the base-class reside in the main eve_sqlalchemy namespace, or something different than 'declarative' to avoid confusion with sqlalchemy itself (I actually followed that lead).
• base class name. Maybe CommonColumns, or EveBaseModel, or something else would be more appropriate?
• What if the user changes ETAG, CREATED, UPDATED settings in the main Eve app?

Again, I am not confident enough with the extension code, so careful review would be appreciated.