Changing prototype attribution to extend instead overwriting

Now, `Plugin.prototype` is extended instead of overwritten and
all methods are accessible through the following:
* `$.fn.eNestedSortable('method', args...)`

Quickfix to CSS on example

Now, the list numbers are shown inside the element
so a small glitch while dropping over a number is fixed
Merge pull request #6 from edygar/master

Preparing for some future features