Add common const cron expressions in a class that could be extended.

[ptjuanramos]

Adding the follow most common cron expressions:

public const string Daily = "0 0 * * *";
public const string Hourly = "0 * * * *";
public const string Weekly = "0 0 * * 0";
public const string Monthly = "0 0 1 * *";

Written in a non sealed class in order to make the class extendable.