Add invitations (fix #77)

Adds an invite system for only admin users (bob@mynameisbob.com
for now) to use. Send an invite via /invite as an admin; accept
one from /use-invite?INVITE_TOKEN.

Currently creates a user account based on email used to sign up.

When you use your invite, it simply asks for you to set a password
then signs up you. See `views/devise/invitations/edit.html.erb` to
customize that template.

After account creation you're sent to the dashboard, just like a
regular sign up.

Regular sign up is removed, as is Facebook/Twitter sign up/login
until invites are out.