Install only necessary jqplot plugins
Improve custom JSON encoder

* Rename class
* Handle date objects
* Add it to IndicoFlask
Add plugin hook for adding vars in vars.js

Also improve plugin_hook to handle no returned values
Generate vars.js cache file depending on plugins
Remove old statistics plugin
Add plugin_url_rule_to_js helper
Make url_rule_to_js consistent with url_for
Post-WIP fixes
Add logger to IndicoPlugin