Add wp_cache_switch_to_blog.

Taken from
http://plugins.svn.wordpress.org/memcached/trunk/object-cache.php.
Add switch to blog tests from core.
Update references to the object cache.
Remove method check and define static vals.
Call  into local scope.
Update test of global group with switch to blog.
Merge pull request #29 from tollmanz/add-switch-to-blog

Add wp_cache_switch_to_blog.