Merge boostbook from trunk.

- Better support for attributes on root elements - including `lang`.
- Add missing copyright to some files.


[SVN r67666]

tools/boostbook

@@ -1 +1 @@
-Subproject commit 560f5c529e45adc334be1b1a5635425e2700aad3
+Subproject commit e755a0fafb76ab16b634f3d9c90e59bd492861c1