Added site-wide tracking

Hooks on to page-header template hook from core
Fix relative imports
Merge branch 'imp-sitewide' of https://github.com/pferreir/indico-piwik into pferreir-imp-sitewide

Conflicts:
	indico_piwik/__init__.py
	indico_piwik/forms.py
Merge branch 'pferreir-imp-sitewide'