zpm: fix a python 2.7.2 encoding issue

With python 2.7.2, there was a problem with tar'ing the rendered
index.html template, due to a bug with unicode/cStringIO

See http://bugs.python.org/issue1548891.
Merge pull request #83 from larsbutler/fix-html-encoding

zpm: fix a python 2.7.2 encoding issue