Change OnBeforeTraverse and OnAfterTraverse signatures to use DirectoryInfo objects

[plastikfan]

The functionality introduced in issue #190, was based upon the passing of a string path. This is not very useful as it mean the client has to invoke Get-Item on this path to get the DirectoryInfo instance. This is just wasteful, because the DirectoryInfo instance has already been obtained so its a bit pointless to require the client to re-obtain it. When a large directory tree is being traversed, this will likely add a noticeable and un-necessary overhead to the runtime.

Change the signatures to use DirectoryInfo instead of a string Path.