WebStat: Enable custom summary

* Adds the dependency matplotlib.

* Fixes the fact that path was always printed.

* Removes annoying try except.

Signed-off-by: Guillaume Lastecoueres <guillaume@tind.io>
Shorter version:
```python
out += """</table></div>"""
if path:
    out += """<div><img src="%s" /></div>""" % cgi.escape(path.replace(CFG_WEBDIR, CFG_SITE_URL))
```