Update Permission.can_access to use new permission service

The old permissions API for checking authorization to a particular
product was over-complicated because it exposed an RDF response when a
simple boolean response was sufficient.  We should now upgrade to the
new API.

Use the new API by:

* Adding config.oreilly.permissions_service_url
* Extract an HttpApi module from RdfApi that wraps Net::HTTP without RDF
* Change can_access and permitted_products to accept objects rather than
  strings.  can_access needs multiple fields from the Product, and
  authorize! probably shouldn't need to know the details of which user
  fields are passed to the API.

Leave permitted_products to use the old RDF API because there is no
replacement for it (yet).
Improve error message to include ISBN when not authorized

We're changing the product that confers access to the paid_content page,
so anyone who was setup with permissions before will need to get new
media permissions for the new title to see the authorized use case.

Make it easy to know which product to add by supplying right in the
error message.
Switch the sample product used to confer access to paid_content

The new API development endpoint doesn't know about the book we're
using (Enterprise Rails), so it's hard to develop with.

Switch to using a book that:

    (1) the development endpoint knows about, and
    (2) has the same ISBN in production so that OPMI lookups still work.

"MapReduce Design Patterns" fits the bill.
Hide HTTP 404 errors within the Product#title accessor

Some products we get from the Permissions#permitted_products API are
not published in OPMI, so we cannot get their titles.  We currently
raise an unhandled exception in this case.

Handle the exception by treating it as a nil title: this kind of
accessor should probably not leak HTTP exceptions.
Generalize log messages in fetch_graph

RdfApi#fetch_graph used to be just for Permissions, but now it's also
for OPMI.  Its log messages mention permissions specifically.

Remove the reference to permissions in favor of mentioning RDF graphs.
(Also correct a capitalization error.)
Configure openid_service_url in development environment for now