*
@method last([num] = 1)
@returns Mixed
@short Returns the last element(s) in the array.
@extra When <num> is passed, returns the last <num> elements in the array.
@example

[1,2,3].last()        -> 3
[1,2,3].last(2)       -> [2,3]

*
function(num) {
      if(isUndefined(num)) return this[this.length - 1];
      var start = this.length - num < 0 ? 0 : this.length - num;
      return this.slice(start);
    }