hdf5doc

A very rough draft of the following sections for 1.8 format:
1. Version 2 superblock in IIA
2. Superblock Extension  (IIC)
3. Version 2 B-link Trees and B-tree Nodes (IIIB)
4. Versoin 2 Data Object Headers (IVB)
5. New messages for 1.8:
   Link Info Message
   Link Message
   Group Info Message
   B-tree K values Message
   Driver Info Message
   Attribute Info Message
   Object's Reference Count Message
6. Add version 3 for Shared Object Message