Report file name in error messages in Pod::Tree::Node

add non-POD option to Pod::Tree (see Mail/jvromans)
write Pod::Tree::Source         (see Mail/jvromans)

pods2html
Is there some way to make --base default to HTMLDir?
Is this the Right Thing?

Move the get description code from Pod::Tree::PerlPod and 
Pod::Tree::PerlLib down into Pod::Tree::PerlUtil.

