Added a Bookmark model and an app.bookmarks collection.
Changed populateBookmarks so that it searches through app.bookmarks via tag,
rather than directly using the results of an api call.
Reworked the app so that it now grabs all of the bookmarks from Pinboard in one shot instead of on a tag-by-tag basis. This means that the app is much more responsive when clicking on a label, and also now provides us with the list of bookmarks we need for searching.