Add methods for text rendering and font handling

- Paper.printTransformed: Renders text letters with a transformation
callback.  Can accept font options as an object.  Intended for use
with wrapper functions that exploit the callback ability.

- Paper.printCircle: Renders text aligned to a circle path.  Uses a
Paper.printTransformed callback.

- Paper.printJustified: An improved version of Paper.print.  Has
horizontal and vertical alignment options.  Uses a
Paper.printTransformed callback.

- Raphael.registerFont: Now returns font after registration, for use
in custom Cufon registration handlers.

- Raphael.getFontList: Retrieves an array of registered fonts.

- Raphael.matchFont: Retrieves a registered font.  Intended to
complement Raphael.getFontList.

- Raphael.chord: Utility function to calculate the angle of a chord
given the chord length and circle's radius.  Used in Paper.printCircle.