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

# Add support for 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.

![Screen Shot 2013-01-17 at 4 39 15 AM](https://f.cloud.github.com/assets/251435/72656/d714b0b2-601c-11e2-9d82-e96df04ecacc.png)

# Make labels in the candidate window 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 under '-updateCandidates:andComments:withLabels:highlighted:'
in 'SquirrelPanel.m' have been refactored.

![Screen Shot 2013-01-17 at 4 38 26 AM](https://f.cloud.github.com/assets/251435/72650/b2fd4838-601c-11e2-9890-e2c44cec8890.png)