Create invitations table in db

* required string token

[#14]
Add Invitation model

* generates random token before it saves

[#14]
Implement SignIn model

* It needs an omniauth auth hash and an invitation token
* It has a simple public interface: #user
* First it attemps to look up the user using the auth hash. If that
  fails, it verifies that an invitation exists with the provided token.
  If that succeeds, then a user is created with the provided auth hash.

[#14]
Add methods to User model

* These are needed satisfy the API expected by the SignIn model

[#14]
Nitpick URI consistency

* Yes, I have a problem.

[#14]
Persist given invitation token through flash

* Adds helper method to get the invitation token, first from the params
  and then from the flash

[#14]
Refactor authentication design to use invitations :)

* Now new users must provide a valid, unused invitation token in order
  to have their account created in Snapme. Ballin'

[#14]
Add invitation_requests table to db

[#14]
Move alerts, add '.disappear' class for fade outs

[#chore]
Configure mailer in development and production

* using mailcatcher for development mail

[#14]
Add InvitationMailer

[#14]
Clean up some translations and organize routes

[#chore]
Add invitation request routes and translations

[#14]
Add InvitationRequest model

[#14]
Implement InvitationRequestsController and views

[#14]
Add welcome message and link to request invitation when logged out

[#14]
Add pending invitation request count to team views

[#14]
Make InvitationRequestsController explicitly authenticate

* Rather than adding auth as an afterthought to this protected
  controller, let's make it explicitly authenticated and remove auth
  from select actions.
* This seems like a safer habit as you're more likely to accidentally
  require auth where it's not needed than the opposite
* Also this catches the sublety that we were missing auth on the destroy
  and send_invitation actions. Whoops ;)

[#14]
Update views with rel handles to grab in specs

[#14]
Config mailer default url host for test environment

[#14]