- renamed empty filter to Null filter
- columns are now renamed recursively, fixing bug in not valid XML code when the columns still had a numeric name
- added a new parameter in API for Referers module: expanded. If set to true, it will load the data recursively (eg. all keywords + the search engines for each keyword).

git-svn-id: http://dev.piwik.org/svn/trunk@482 59fd770c-687e-43c8-a1e3-f5a4ff64c105