Fix method declaration, error in PHP 5.4

It was fixed in http://www.phing.info/trac/ticket/555, but the
sfPropelPlugin bundled in sf 1.4 is dated.
Clear the search index using ES API
Add unique update() method
Use new ES API methods
I don't think this is a good idea anymore
Throw error if db i18n languages are not available
Add specific flush method to clear index
Clear the search index from the purge task