Use middleman HEAD

To use 3.1.0
Add timetable frame
Add session detail page template
Add link to session detail page
Use partial for schedule
Add empty schedule
Add empty sessions.yml
Add simple detail page
Show gravatar icon
Extract helper method about gravatar
Add session data
List schedule
Add sessions to schedule
Update bundle
Remove "comming soon"
Remove unnecessaru guard from missing session id
Remove unnecessary
Add events
Use key as a data type
Add keynote speakers