Issue #266 - Add Link parsing to IssueCollection.

We're pulling the page number out for next, prev, and last relations to
be used by the View for requesting the correct set of issues for
pagination.
Issue #266 - implement requestNextPage and requestPreviousPage (plus
some extra comments).

I expect this to be rewritten pretty soon--it will probably need to
change once we add the extra complexity of sorting.
Issue #266 - logic for disabling next/previous page links.
Issue #266 - remove filesystem caching for /issues view.

This breaks pagination for some reason I don't understand yet. But we're
doing conditional requests now so it should probably be OK.
Issue #266 - Add pagination support to /issues endpoint.