feat(py3k): adds Python 3 support, fix README error

In order to support python 3 support for simpleflake, the following
changes are made:

- use absolute imports

'from simpleflake import X' does not work on Python 3.

Additional fixes:
- rename README.txt to README.md so it can install using pip
- fixed imports in tests

Other changes:
- Add python 3.3 as a classifier
- move test.py to tests/
- move simpleflake.py to simpleflake/
- rm Makefile - just use nosetests
- Added __init__.py where needed