Merge pull request #356 from untitaker/sprint-branch

Cookies and so on
Fix uri_to_iri on 3.x
iri_to_uri should be idempotent

It isn't but let's fix this later.
Make uri/iri conversion functions idempotent
Use unittest2 on travis
Add PyPy to tox and travis again
Consistently use lowercase asserts
Get rid of unittest2 dependency
Enable notifications again
Remove six dependency
Better __str__ handling
Remove six reference

Apparently this module is untested.
Remove last six references
Small micro-optimization for datastructures