include acces_tokens in web requests

[mbjones]

Secure operations to write endpoints and read endpoints that need to establish identity will need to set the Authorization header in the http request. Standard R libraries like httr support setting these headers, so we should just write a utility method that might optionally include the authorization header if a user is logged in and they are accessing an endpoint that requires auth. The header will have the format:

Authorization: Bearer <a-long-token-string-goes-here>

The access token would be tetrieved from the "session" or token storage system described in #65