cStringIO deprecated in Python 3

[rjleveque]

src/python/visclaw/JSAnimation/html_writer.py uses cStringIO, which has been deprecated in Python 3, see e.g. http://stackoverflow.com/questions/28200366/python-3-4-0-email-package-install-importerror-no-module-named-cstringio

@ketch: I think we dealt with this someplace else in Clawpack already with a good way to deal with the different versions, but I can't remember where now...

[ketch]

This is more difficult than I expected. I have a version that runs here:

https://github.com/ketch/visclaw/blob/python3_html_writer/src/python/visclaw/JSAnimation/html_writer.py

However, the animation images and the control icons just show up as broken links, so this is useless so far.

[ketch]

This got fixed in 3f9b25a. Closing.