Be more explicit about comparing

Only instances of identical classes should be compared.
Introduce new Attribute class
Add support for default factories
Add changelog entry
Make the teaser use Attribute
Add Rich & Julian for their feedback
Two digits should be enough for everybody
Doc polishing
Add default_value too

It's about convenience after all.
Add sphinx version tags
Use the much nicer default_value for teaser
Move main attribute up
Move tutorial-ish above API docs.
Call factory only if the kw was really missing
Annotate examples in docs
Minor polishing
Only accept NOTHING instead of instances

Turns out, isinstance is slow like molasses.