NEW: CMS site tree status icons

Added coloured status 'dots' to overlay page icons and improved visibility of status labels. As discussed in https://github.com/silverstripe/silverstripe-framework/pull/2794

![img](https://cloud.githubusercontent.com/assets/1079425/5371903/366f3704-8090-11e4-9481-16d9e81a0684.png)

Ah right, there you go, learned something about SASS today :)