Adds scope 'admins'

Issue #33
Creates no_admins scope in Role model

Issue #34
extract hardcoded values off session invitation
use draper decorators
extract hardcoded values off reminder and spots_available emails
remove digital science logo attachment
Merge pull request #34 from pvcarrera/master

Adds scope 'admins'
Merge pull request #37 from despo/tidy-up-email-templates

Tidy up email templates
don't serve sponsor images as assets
Merge pull request #38 from despo/move-sponsor-images

don't serve sponsor images as assets
add pivotal image