fix relative import issue for python3

core needs to be .core or idna.core - using explicit name to make it clear where it comes from.
fix more issues with python3 port

fixed issue with return type of .encode (now bytes) and expectes bytes
as agument to .decode (tho python2 code can fudge this).
add tests/__init__.py for test discovery

with unittest
tweak decode handling for python2 u -> u cases
Merge pull request #2 from underrun/master

Improvements to Python 3 support (Thanks!)