add .path property to createjs.LoadItem

[OJayRobinson]

LoadQueue._createLoadItem is setting a path property on a LoadItem (line 1365), but LoadItem does not include a path property in its definition.

I suggest we add it.

[lannymcnie]

I believe this is an auto-generated value that is cleared before the load is complete. I don't know if it makes sense to doc it.

[OJayRobinson]

I'm not sure if it is cleared, but it is used by other classes which is why I would recommend it be doc'd