Allow cronExpression to be an instance of the CronExpression class

Allow the trigger parameter 'cronExpression' to be either passed in as a String
or an instance of the CronExpression class.

This is used to allow a different timezone to be set for the quartz jobs, which I couldn't find a current way to do this. Currenly I run grails as UTC to force all dates to be stored as UTC, however we still needed to set a different timezone for the Quartz jobs.

Example:

```groovy
    static triggers = {
        def expression = new CronExpression('0 0 21 ? * *')
        expression.timeZone = TimeZone.getTimeZone("Europe/Dublin")

        cron(cronExpression: expression)
    }
```