new feature: links associated with apps.

press the app icon on the main screen and you're taken to a links activity. you can add and remove links there.
added more debug logging for skipped apps
added description and changelog to AppInfo
fixed typo, columns were not added when creating from scratch
get last market update date
added AppDetails to store description, etc.
linked Links to AppDetails with FK
Merge pull request #351 from nelenkov/links-page

Added AppDetails table/class; linked Links to AppDetails with FK