Display singular interval units as appropriate.

Modernize a bit, too.
Move Gestalt system version checks into one place.
Work around NSComboBox baseline misalignment in 10.10.
Adjust fake combo box menu for yet another set of metrics in 10.10.
Remove remainder of accommodations for compilation without NSUserNotification* defined.
Make tests run again by setting OBJC_DISABLE_GC.

Still need to migrate to xctest.
libParseDate changes for 10.9/10.10 (Perl 5.16).

10.7 is now the minimum OS, so we require final compilation with Xcode 6.1 on 10.9 with the 10.10 SDK.
Update Sparkle to 1.7.1 + changes for continued 32-bit compatibility.

Newer Sparkle versions require the modern runtime because they use ARC.