IE 8 compability, replacing old method

For us who still have to support IE 8, the datepicker are having trouble with determine the year, due to the old getYear() + 1900 stuff. I have replaced it with getFullYear for the picker to work in IE 8, 7 as well.
![ie8_bug](https://f.cloud.github.com/assets/789341/476719/ae0f4374-b7c6-11e2-8489-f69b65ff22fe.png)