Added dir attribute to body tag from directionality setting

Hi there,

This is a simple change to enhance the way in which the directionality setting is being applied to popup dialogs.
It sets the dir attribute on the body tag instead of just setting the dir property.
Setting the attribute has the effect of setting the property as well.
The reasoning behind this change is to allow us to deal with accessibility issues and style requests for those using right-to-left languages through CSS.
I suppose another argument in favor of this change would be that the main editor iframe also sets this attribute on its body element so this would help provide a consistent environment/

I've tested this change on my local machine but have not tested this thoroughly on a range of browsers (notably this has not been tested in IE).
As this change is modeled on how the directionality setting is handled for the main editor iframe I don't suspect there are any issues.

This is my first proposed contribution to the TinyMCE project, I apologise if I've gone about things in the wrong manner or if this change is not suitable or in line with the project and I have wasted anyone's time.

Many thanks
Sam Hemelryk