Ignore .egg-info files when building debian packages.

I was having a problem where py2dsc was dying because
it was trying to generate a diff that was including the
.egg-info directory setup.py was creating.

It turns out Arnaud Fontaine <arnau@debian.org> had the same problem
and suggested a fix -- to add extended-diff-ignore to debian/source/options

Fixes deb#657611
Merge pull request #57 from detrout/egg-info
fixup 'extend-diff-ignore' to ignore .egg-info files