Make javascript extendable and refactor success handler

All javascript methods can now be overridden by adding a file to your project and extending the core handler.

The javascript file is called "jquery.inplaceeditform.hooks.js" and should contain calls to $.inplaceeditform.extend adding new functions or replacing core ones.

Example (show message in bootstrap modal):
```javascript
$.inplaceeditform.extend(
    {
        inplaceApplySuccessShowMessage: function(inplace_span) {
            var self = $.inplaceeditform;
            if (self.opts.successText) {
                var modal = $('#inplaceedit-modal');
                var title = modal.find('div.modal-header h3');
                var body = modal.find('div.modal-body p');

                title.html('Success');
                body.html(self.opts.successText);

                setTimeout(function () {
                    modal.fadeOut(function () {
                        $(this).remove();
                    });
                }, 2000);
            }
            modal.show();
        }
    }
);
```