Refactor SquirrelPanel using Core Text & Support customizable horizontal/vertical spacing

# 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.)

# 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_ specifies the scale that multiplies the default horizontal spacing. (default: 1.0)
- _vertical_spacing_multiplier_ specifies the scale that multiplies the default vertical spacing. (default: 1.0)