Add a convenience method for reading "all text values of all child elements"

[cowtowncoder]

A relatively common case seems to be that of mapping structures like, say:

<root>
  <elem1>text</elem1>
  ...
 <elemN>textN</elemN>
</root>

Iterating over this structure is not super hard, but it would be nice to just have a method for "reading it all in", to produce map like:

{ "elem1" : "text", ... , "elemN" : "textN" }

and then let caller do whatever it needs to. Textual content should probably be simple collected with 'collectDescendantText', to work even if sub-elements were included.