Refactor initialisation

This reverts storing initialised `Expanding` instances in `data` (rather than in a registry) as it leads to more understandable code. (Takes some inspiration from twitter bootstrap plugins.)

It also removes the `$.expanding.opts.update` option. This is used to provide a default callback on update, but is not documented, and is a little confusing. Note: `$('textarea').expanding({ update: function () {} });` is still supported.

`.jshintrc` is updated, and .js files have been updated to reflect these changes.