Support OS X Notification Center.

For OS X Mountain Lion, notifications could be optionally delivered
through OS X Notification Center instead of Growl by setting a boolean
variable 'show_notifications_via_notification_center' in 'squirrel.yaml'
which is detailed below.

Added a boolean variable 'show_notifications_via_notification_center'
in 'squirrel.yaml'.
When this variable is set to 'true', 'show _notifications_when' is not
set to 'never' and the OS version is at least 10.8,
notifications will be delivered throug OS X Notification Center instead
of Growl.
Labels before candidates is now customizable.

The labels in the candidate window can now have
customized font, font size, text color and
highlighted text color.

Four new options including 'label_font_face',
'label_font_point', 'label_color' and
'label_hilited_color' have been added
to the 'style' section in 'squirrel.yaml'.

'label_color' and 'label_hilited_color' might
be moved to each scheme under the 'color scheme'
section in the future.

A few lines of code in 'SquirrelPanel.m' under
'-updateCandidates:andComments:withLabels:highlighted:'
have been refactored.
remove invisible trailing CR in utf8/checked.h
Merge pull request #10 from lembacon/master

Add support for OS X Notification Center & Make labels in the candidate window customizable