Added waitfor task to the relax ng file

unfortunate there is no entry for waitfor task inside the relax ng file. so i decided to implement one by myself. can someone please review this pr specially for the `<ZeroOrMore>` elements where i am a little bit unsure.
hope you like it and thanks in advance
Update phing-grammar.rng
Merge pull request #287 from siad007/patch-1

Added waitfor task to the relax ng file