accept more than 2 parameters in CONCAT function

The DBAL Platform supports more then 2 parameters but the ConcatFunction only validates 2 parameters to CONCAT. This commit allows to pass more than 2 parameters to CONCAT. Also this change would require that `getConcatExpression` accept array as a parameter. I have opened a pull request for that as well.

Here is the pull request : https://github.com/doctrine/dbal/pull/275

I also propose that the function `getConcatExpression` only accept array of string rather than multiple string arguments.