Refactor SquirrelPanel using Core Text.

The text rendering part in SquirrelPanel has been
refactored using Apple Core Text technology. More advanced
candidates/labels/comments layout options could be implemented
in the future.

Core Text is a new technology and much faster comparing to
Cocoa text rendering sub-system (which is based on ATSUI,
and ATSUI has been deprecated starting OS X 10.8).

This also fixed a bug when font cascade list is being used, the
text height is miscalculated and thus the text is not
vertically aligned.
(See the screenshot in #14 for more information.)
Use bzero to clear SquirrelUIStyle struct.
Fix initial alpha value for last commit.
Support customizable horizontal/vertical spacing.

The horizontal/vertical spacing in the candidate window
is now customizable.

Two new options has been added to the 'style' section
in 'squirrel.yaml' (can also be used under each color
scheme section):

- 'horizontal_spacing_multiplier' is the scale that will
multiply the default horizontal spacing. (default: 1.0)

- 'vertical_spacing_multiplier' is the scale that will
multiply the default vertical spacing. (default: 1.0)
Remove trailing whitespace in candidate format.

The trailing whitespace in the default candidate format
has been removed, since SquirrelPanel has already calculated
and appended horizontal spacing as of the refactor using
Core Text.