New feature: Easier to run multiple wiks off of one set of code. Name

New feature: Easier to run multiple wiks off of one set of code. Name
your logo and signature image files "YourWikiNameLogo.png" and
"YourWikiNameSignature.png" and put them all into
themes/default/images. YourWikiName should match what is defined as
WIKI_NAME in index.php. In case the image is not found, the default
shipped with PhpWiki will be used.

git-svn-id: svn://svn.code.sf.net/p/phpwiki/code/trunk@2893 96ab9672-09ca-45d6-a79d-3d69d39ca109