Image caching -- sorta working? :D

- saving stub image info to individual .plist files as well as in a straight URL list for the article
-- may want to break the list down to per-section if we want to replicate some previous behaviors
- went with shorter filenames (last filename only without path info), may break in some rare corner cases but the long ones were breaking frequently
- using the live session articleStore for caching so it actually uses the right article!

This doesn't yet handle thumbnails in list pages real well, as global cache doesn't know
which article is which.

Change-Id: I923a7f6f2eb58d03edc9d3b7efe68c98595e9acb