First implementation of Invitations model

The Invitation model will enable managers to invite their employees to
join their Ringbase organization using email.

This commit just adds the necessary migrations/models/basic spec - no UI
work is done yet.
Functional organization invitations

The flow for organization invitations is:

  1) Manager signs up, creates business profile
  2) Is prompted to enter agent emails, submits invitations
  3) Agent opens email, clicks link to enter info and complete signup
  4) Agent is added to organization and sent to dashboard
Set default host option
Merge pull request #14 from RingBase/invitations

Functional organization invitations