Python 3 issues

@ejeschke I forgot ... does Ginga support Python 3?

I thought it does, but the tests currently don't pass:
https://travis-ci.org/ejeschke/ginga/jobs/42519643#L263

and I can't start the reference viewer on my machine with Python 3:
```python
$ ginga
Traceback (most recent call last):
  File "/Users/deil/Library/Python/3.4/bin/ginga", line 4, in <module>
    __import__('pkg_resources').run_script('ginga==2.0.20141130031913', 'ginga')
  File "/opt/local/Library/Frameworks/Python.framework/Versions/3.4/lib/python3.4/site-packages/pkg_resources.py", line 517, in run_script
    self.require(requires)[0].run_script(script_name, ns)
  File "/opt/local/Library/Frameworks/Python.framework/Versions/3.4/lib/python3.4/site-packages/pkg_resources.py", line 1436, in run_script
    exec(code, namespace, namespace)
  File "/Users/deil/Library/Python/3.4/lib/python/site-packages/ginga-2.0.20141130031913-py3.4.egg/EGG-INFO/scripts/ginga", line 14, in <module>
    main.reference_viewer(sys.argv)
  File "/Users/deil/Library/Python/3.4/lib/python/site-packages/ginga-2.0.20141130031913-py3.4.egg/ginga/main.py", line 475, in reference_viewer
    viewer.main(options, args)
  File "/Users/deil/Library/Python/3.4/lib/python/site-packages/ginga-2.0.20141130031913-py3.4.egg/ginga/main.py", line 260, in main
    from ginga.qtw.GingaQt import GingaView
  File "/Users/deil/Library/Python/3.4/lib/python/site-packages/ginga-2.0.20141130031913-py3.4.egg/ginga/qtw/GingaQt.py", line 32, in <module>
    from ginga.qtw import ImageViewCanvasQt, ColorBar, Readout, PluginManagerQt, \
  File "/Users/deil/Library/Python/3.4/lib/python/site-packages/ginga-2.0.20141130031913-py3.4.egg/ginga/qtw/ImageViewCanvasQt.py", line 12, in <module>
    from ginga.qtw.ImageViewCanvasTypesQt import *
  File "/Users/deil/Library/Python/3.4/lib/python/site-packages/ginga-2.0.20141130031913-py3.4.egg/ginga/qtw/ImageViewCanvasTypesQt.py", line 15, in <module>
    from ginga.canvas.mixins import *
  File "/Users/deil/Library/Python/3.4/lib/python/site-packages/ginga-2.0.20141130031913-py3.4.egg/ginga/canvas/mixins.py", line 4, in <module>
    from CanvasObject import *
ImportError: No module named 'CanvasObject'
```