[WM] <sitetree> tag

[Justin Mason]

... is now implemented!

BTW the really neat thing about this, is that I've implemented it as a
PerlLib file; the WebMake file includes this line:

	<include file="${WebMake.PerlLib}/sitetree.wmk" />

and that file contains code to define a <sitetree> tag that can be used in
the WebMake file.

Also, everything that the <sitetree> implementation needed to do was
possible using the PerlCodeLibrary methods -- no delving into WebMake
internals (after I added a few methods).

Check it out -- it's on the website (in the devel version).  I'll be
releasing 1.1 (hopefully) sometime in the next week, with this new
tag-defining stuff, and more diagnostics...

Cheers,

--j.