User may see their pictures

This commit introduces the concept of nested resources. Nested resources
are a nice way to denote ownership. By default, nested resources just
use the top level controller. Because I expect the pictures index to
wind up showing all the public pictures, I decided to make a
controller explicitely for the users pictures.

You *could* put the new/create methods in the users pictures controller
as well, and it wouldn't hurt anything. Since pictures aren't likely to
be created for anyone *except* the currently logged in user, I see it as
reasonable to leave it within the pictures controller.

We also get to see the new html5 `figure` and `figcaption` tags!
Redirect to users pictures when picture is created

Instead of redirecting to the home page, let's redirect to the users pictures
page! Such UX, Very Workflow. Much design
Extract have_picture matcher

To much duplicate code!  Custom matchers are useful when you want to
make an assertion about your applications logic.
Merge pull request #3 from codeunion/user-may-see-all-their-pictures

User may see all their pictures