User Notification Center Support

Users  Notificaiton Center when available. Falls back to Growl.
First working cut of magnets

This iteraction is currently dependent on dn inclusion in the magnet.

It would be better to use the namespace from the rss feed for both
magnets in files. This requires significant changes to the feedchecker
and downloader, and migration from older preferences formats.
Storing Title in the Downloaded history

Storing title in the download history enables the recently downloaded
list to show the real name. It also provides a reliable name for
magnets.

To Do: Handle migration of old preferences file formats.
Migration of downloads history to new format

Change old array of strings to new dictionary format when detected.
Reverting xib changes.

Removing text changes as requested by Georgio.
Reverting deployment target change.

Unclear if this actually makes any difference.
Improved History Migration

Migrating with filename to keep parity and ensure only short labels
appear in the menu.
Changing key for downloaded files to History

Since the data type changed in the user defaults, it's better to change
the key.

Also fixed a typo.

NOTE: If you happened to have built from the previous commit, you'll
need to delete your preferences, or at least the downloadedFiles key.
Putting in a special check there, just for me seems like a waste of
code.
Removing errand tab character.
Simplified User Notifications with Better Text

One piece of notification code for files and magnets. Notifications now
use title instead of url, which is much more readable.
Merge pull request #15 from studiosciences/master

Magnets, User Notification Support, Recents with Titles