Fix destroyWarningDialog() function removing other jQuery dialog instanc...

Fix destroyWarningDialog() function removing other jQuery dialog instances.
Destroy jquery-idleTimeout warning dialog using the specified ID when initialized instead of ".ui-dialog-content" class which also destroy other jquery UI dialog instances.

Had an issue where clicking on the "Stay Logged in" button remove other jQuery dialog instances (both open and closed dialog) because the destroyWarningDialog() function remove the session timeout dialog using  ".ui-dialog-content" class selector which is used by other jQuery dialog instances.
![dialog-destroy](https://cloud.githubusercontent.com/assets/1471939/4027498/a5e045b6-2c2f-11e4-9244-87e72c4b95ef.png)