[HyperDB] Big update in SVN

[Andy Skelton]

I just committed a major update. HyperDB now works with WP trunk by
default, and it works with WPMU trunk with only a minor config change.
The installation directions have been updated.

The common stuff is abstracted into a class named "db". The "wpdb"
class extends the "db" class, adding the appropriate methods for WP or
WPMU. This was done because of differences in the way WPMU prefixes
table names.

There are still problems with the get_ds_part_from_table method using
prefixes. If you use that feature, tread carefully.

My tests were run on fresh trunk checkouts with a single database
server. Advances features haven't been tested. As always, test this on
a development platform before deploying to a production site. No
warranty, remember? :-)

Andy