Support python 3

Please add support for python 3.x.

``` python
>>> from swf.movie import SWF
Traceback (most recent call last):
  File "<pyshell#1>", line 1, in <module>
    from swf.movie import SWF
  File "C:\Users\username\AppData\Local\Programs\Python\Python35-32\lib\site-packages\swf\movie.py", line 4, in <module>
    from tag import SWFTimelineContainer
ImportError: No module named 'tag'
```
