Add a getInstance method to the base SirTrevor object

This makes it easier to access instances rather than always going through the `instances` array.

The `getInstance` method works as follows:

* If no params, return the first item in the array
* If a string is given, use that to lookup by the `SirTrevor.Editor` ID
* If an integer is given, return the array item at that position

cc @ninjabiscuit @andrewsprinz