HOTFEATURE: New Merge Codes For Themes

Added 2 new merge codes that will be useful for making themes:
##THEMEDIR## = "/Shared/Themes/[ThemeName]/"
##THEMEIMGDIR## = "/Shared/Themes/[ThemeName]/images/"

This saves you from having to type out the entire directory structure when including files in your theme.

Note:This will not work inside a CSS editor because css files are not processed by the merge code system. In a css file you can simply use "../images/logo.jpg" to accomplish the same thing as ##THEMEIMGDIR##