- Limit the new index size to ensure we don't get too big on UTF8 databases.
- dbDelta doesn't handle index changes well so instead add a new index and drop the old one manually.
- Updated admin manual.

git-svn-id: https://plugins.svn.wordpress.org/wp-statistics/trunk@992641 b8457f37-d9ea-0310-8a92-e5e31aec5664