tabs: allow negative indexes

tabs: allow negative indexes

while looking at the tabs code, I noticed that it is currently using the numeric value -1 to identify that no tab is selected. a few points in the code special-case -1 in the index.

But, arrays can work with negative indexes. With very little effort the code can be made to accept negative arrays. This would cause the code:  
  1. $('#tabs').tabs('select',-1); 
To select the last tab.

I propose to stop special-casing -1, otherwise negative indexes aren't possible. 

My primary candidate to special-case the index is NaN, which is also a typeof number. Otherwise it can be any other type.