responsive to all devices.
fixed the global nav on top when scrolling down.
Merge branch 'master' of github.com:pragmaticly/rubyconfchina.org

Conflicts:
	source/css/main.css
update mailto
update Images
resize speaker's avatar for alignment.
make the logo visually aligned center(actually the bg photo is not perfectly centered).
retina ready all icons.
The logo need to adjust back to pixel center aligned when on mobile devices.
forgot to get rid of livereload for production.
Fix kevin's last name
Merge branch 'master' of github.com:pragmaticly/rubyconfchina.org
Merge remote-tracking branch 'rubyconf/master'