Version Control With Subversion Source

* en/book/ch07-customizing-svn.xml
Add docs for the new 'diff-extensions' runtime configuration option.

* en/book/ch08-embedding-svn.xml
Fix reference to "libsvn_ra_dav". Remove the "Inside the Working
Copy Administration Area" section (and a crossreference to it).
Update Python example to demonstrate callbacks/batons, and add text
around this. Add "Functions and Batons" section.