Added disable and enable functions to combobox.

Combobox can now be disabled by calling .combobox('disable'). I basically disabled the input element, removed the click event for the select element and disabled the element as well.

The combobox can be enabled back by calling .combobox('enable').

I noticed that calling the .combobox() function the second time would consider 'input.combobox' as a valid element, so I had to avoid it in the library.

Please let me know if I can improve the code somehow, as this is my first open-source contribution :D