Updated requirements.txt and link rewrite change

Updated requirements.txt to include Jinja2 and remove pyinotify. There did not seem to be a dependency on pyinotify as far as I could tell. Please verify. Jinja2 is of course the template engine.

Tested on Mac OS X 10.8 and Linux Ubuntu 13.04.

Another modification:

When I change the format for permalinks in site.yaml, to for example:

permalink_format:
post: '{{year}}/{{month}}/{{slug}}'

(which is a fairly common format), I still want to the img src re-writing to work when the post is displayed from the front page. Previously it was assumed that only the slug would be used during re-writing, but now I use the permalink for the re-write instead. Does that make sense? Nothing breaks if the standard setting:

permalink_format:
post: '{{slug}}'

is used instead.