Update library to work under IE8

Hi,
I am currently work on project where we are using Your library for switching between languages, everything looks cool, except that someone forgot to test this on IE8 because IE8 is the target browser for this project. Unfortunately on **IE8** library does not work, because we got error **'Object does not support this property or method'** on line `this.langDefaultText = this.data` in **_getTestNodes** method. The problem occur because `langDefaultText ` was the custom property for Text Node, and it turns out that such an assignment is not possible in IE8. So I decided to make few changes to work on IE8. Please review.