Extract Touchstone session management to its own controller

Provides two endpoints for creating a new session and processing the
session after it has been created by Shibboleth locally on the server.

The endpoints do not adhere to REST conventions since it uses a `GET`
request to create a user session. This is an artifact of the
request/response cycle of authenticating via Shibboleth. After correctly
authenticating with the identity provider, the identity provider
correctly `POST`s back to the Shibboleth handler on the server to create
the session.  After the session is created, it redirects back to the URL
that initiated the request for authentication. It is at this point that
the Rails application can sign in the user using the `REMOTE_USER`
environment variable in Apache.

Addresses #18 by isolating the endpoint that signs in the user using the
`REMOTE_USER` variable to a single, unique endpoint.