theme: pure theme

sorry for long response time for this PR; theme's license is in theme/LICENSE
this modified theme is still not ready, even for `staging`, but many templates are already functional, like index page (index page would need some side bar for twits for example, and pictures per each event are missing from content therefore I did not implement that feature yet)
Could you please use the pelican-page-order plugin (already included in pelicanconf.py).