Python 3 support

Makes the codebase compatible with Python >=2.6 and >=3.3. 

- Depend on schematics>=1.0 and marshmallow>=1.0.0-a
- Port code in responder.py
- Add tox and Travis-CI support

Partially addresses #5 (marshmallow and schematics are still hard dependencies).
Subsumes #6 .