Cache the newspaper issue list

Use Drupal's Cache API to cache the list of newspaper issues. Preliminary testing on a newspaper with about 200,00 pages shows that caching the issue list can result in page render times that drop from over 30 seconds for the uncached version versus about 50-60 ms for the cached version. I'll wait until after the 7.x-1.7 release to open PRs, but if anyone wants to test they can get the code from https://github.com/mjordan/islandora_solution_pack_newspaper/tree/cache_issue_list.