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.

[dkellner]

You're right, most users will want to use something like this anyway, so we should provide it. But the drawbacks with changed settings are valid, too.

• I'd leave it in declarative.py, but import from eve_sqlalchemy would be nice.
• For me, DefaultBaseModel fits better as it conveys it's not applicable for any use case.
• Now that the docs will not include the definition of CommonColumns anymore, they should mention all columns the new base class provides, including the warning that changing the above mentioned settings will make an own base class necessary.