Update index.js

The swap method has been updated, for be able of to swap two elements using the keys, the tomes, or any combination of them.

Now is possible to make:
var myArray = [ 'bread', 'bacon', 'lettuce', 'tomato', 'egg', 'cheese' ];
myArrayTome = Tome.conjure(myArray);

myArrayTome.swap(0, 1);
myArrayTome.swap(myArrayTome[0], myArrayTome[1]);
myArrayTome.swap(0, myArrayTome[1]);
myArrayTome.swap(myArrayTome[0], 1);