IIS advanced log importing, netscaler log importing and cs-username handling

Includes changes to the log importer that should fix issues: #6795, #4707, #5980

@quba & @mattab can you take a look and see if these changes will provide the necessary features?
can you add the Netscaler log file to the python tests as well?

Also I see there is a `time-taken` field which I think is important. it is expressed in seconds ([source](http://support.citrix.com/servlet/KbServlet/download/18655-102-665879/NS_AG.pdf))