This patch enables export of books as XML documents.

The XML is DocBook "at the level of structure", but
node contents are wrapped as CDATA, since we
can't be sure that the contents are valid XML.

Several other bugs/feature requests are also
addressed with this patch:

- Fixes bugs

http://drupal.org/node/1898
http://drupal.org/node/1482
http://drupal.org/node/8049
http://drupal.org/node/1899

Should go a long way towards implementing feature request
http://drupal.org/node/2062

It should also be easy to extend this to produce OPML,
for example.

- Adds about 170 lines, of which more than 100 are comments
- Added doxygen comments
- Made doxygen comment format consistent; fixed minor grammatical slips
- A proper Doctype and more informative HTML element is generated
for printer-friendly HTML output.
- Refactored book_print() to use book_recurse().
- Refactored book_recurse(). Applies 'visitor' callback functions to nodes
during weight/title order tree-traversal. The parameterized
visitor callbacks can be used to generate different kinds of output.
There are many other kinds of operations on books which can be implemented
by writing a pre-node/post-node pair of callback functions: word-count/
statistics gathering, comparison, copying, search and replace...
- Introduced book_export() which uses book_recurse() to generate
DocBook-like XML to export book contents in a structured form.
An md5 hash is computed for each node to help import code to
decide if a node needs to be updated or not.