Currently, CloudWatch Events, Application AutoScaling and AutoScaling all have cron expressions, and so they have a Cron helper class. See if we can merge these, and make a more expressive cron expression builder.

At least the syntaxes of AppAutoScaling and AutoScaling are not compatible, they look like this:

cron(0 18 * * ?)    # appscaling
0 18 * * *             # autoscaling

Note the cron() function, and the difference between * and ? for the DOW field.

See comments in #1134