Tabs: Make tabs('url') work as a getter

[pmcelhaney]

I was disappointed to find there's no way to get the URL of the currently selected tab, so I modified the url() function to work as an accessor as well as a mutator. Also added unit tests.

// Existing signature: set a URL
el.tabs('url', 1, '#newurl');

// New signature: get the URL of tab 1
url = el.tabs('url', 1);

// New signature: get the selected tab's URL
url = el.tabs('url');

This is my first attempt at contributing to jQuery UI. Please let me know if I should do anything differently.

[scottgonzalez]

Thanks for the patch, but we're removing the url method. See forum.jquery.com/topic/tabs-api-redesign for more details.