Add support for addressfield.json field.format and field.eg

The 1.0.0 release of addressfield.json (in addition to all the other work gone into this so far) introduces the concepts of field formats (regex patterns for given fields) and example values.

The `field.eg` value is best used as placeholder text. Here, we prepend the example with `e.g.`. It can also be overridden by overriding `$.fn.addressfield.updateEg()`.

It will also conditionally add validation handling via jQuery.validate (if installed).