Print responses from network requests and check for callbacks before executing them
Add tags request method to Pinboard service class
Refactor network code out of PMTagStore

PMTagStore no longer accesses the network. Instead, PMPinboardService
is used to request tags. The app delegate asks the service to download
tags for all usernames when the application becomes active.
PMTagsController observes tag updates and bookmark username changes,
and obtains tags from the tag store.
Move a comment so that Xcode doesn't associate it with the wrong thing
Refactor network code out of PMBookmarkStore

PMBookmarkStore no longer accesses the network. Instead,
PMPinboardService is used to request posts and post bookmarks.
Remove remaining network related code from the bookmark store

PMNewPinTVC now asks the Pinboard service to load previous posts with
the current bookmark's URL, and updates the bookmark's lastPosted
property accordingly.
Merge branch 'refactor-bookmark-posting'