Set up Segment.io by default

In order to analyze metrics later, we need to instrument our app to log
the right metrics. The primary type of instrumentation we care about is
called "event tracking."

We ese Segment.io to capture events whenever possible. It is basically
the adapter pattern for analytics services.

Segment.io lets us drop one JS library into our web apps, or one Ruby
library into our server-side framework, or one iOS SDK into our mobile
apps. Then, we can toggle on and off different services such as Google
Analytics, Intercom, and others.

http://playbook.thoughtbot.com/#instrumentation

If we are working on an app that uses authentication through a library
such as Clearance, we should add the following to
`app/views/application/_analytics.html.erb`:

    <% if signed_in? %>
      <%= render 'signed_in_analytics' %>
    <% end %>

https://github.com/thoughtbot/clearance

At that time, we should also add
`app/views/application/_signed_in_analytics.html.erb`:

    <script type="text/javascript">
      analytics.identify("#{current_user.id}", {
        created : "#{current_user.created_at}",
        email : "#{current_user.email}"
      }, {
        Intercom : {
          userHash : "#{OpenSSL::HMAC.hexdigest('sha256',
    ENV['INTERCOM_API_SECRET'], current_user.id.to_s)}"
        }
      });
    </script>

The most likely first signed in service we'll want is Intercom. It plays
two important roles for an early stage product:

1. communication (and support) with customers and
2. measuring retention (such as Weekly Active Users)

https://intercom.io