Add ability to get font information.

Add three simple font information functions. Usage notes added to README.md.

```
// Return an array of installed font names
macgap.fonts.availableFonts();

 // Return an array of installed font families
 macgap.fonts.availableFontFamilies();

// Return the fonts in the given font family.
macgap.fonts.availableMembersOfFontFamily('Helvetica');
```

As a convention, I've used the same Javascript method names as their cocoa equivalents.