'unique' attribute for tables

[gi0baro]

As @massimo noticed, we missed this: web2py/web2py#395

I think we should start a discussion about this. My points:

• I'm not sure about the use case of this feature
• I don't really like that implementation: if we want this, let's write it from scratch.

What do you think?

[niphlod]

I'm for tackling it when we'll add index support...

[amerikan]

I was about to ask about how would one go about using ALTER TABLE table ADD UNIQUE in PyDal, but it looks like it's something currently not implemented?