python3 support

When I run the tests for python 3:
```python
======================================================================
ERROR: test_participate_errors_when_no_alternatives (djsixpack.tests.class_tests.ParticipateTest)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/joseph/waveapps/src/sandbox/django-sixpack/djsixpack/tests/class_tests.py", line 45, in test_participate_errors_when_no_alternatives
    self.assertRaises(ValueError, expt.participate)
  File "/usr/lib/python3.4/unittest/case.py", line 701, in assertRaises
    return context.handle('assertRaises', callableObj, args, kwargs)
  File "/usr/lib/python3.4/unittest/case.py", line 161, in handle
    callable_obj(*args, **kwargs)
  File "/home/joseph/waveapps/src/sandbox/django-sixpack/djsixpack/djsixpack.py", line 74, in participate
    session = self._get_session(user_agent, ip_address)
  File "/home/joseph/waveapps/src/sandbox/django-sixpack/djsixpack/djsixpack.py", line 47, in _get_session
    return sixpack.Session(**session_kwargs)
  File "/home/joseph/waveapps/src/sandbox/django-sixpack/Sixpack_client-1.1.0-py3.4.egg/sixpack/sixpack.py", line 28, in __init__
    options = dict(default_options.items() + options.items())
TypeError: unsupported operand type(s) for +: 'dict_items' and 'dict_items'

```
this indicates that the client doesn't support 3.0?

Just posting this as @dlanger added python3 to the setup.py (I don't know if that is supposed to indicate support) and python3 isn't in the travis.yml.