Add start_time information to Schedule.to_s() method

Hi,

This is a small patch to add start date information to the ```IceCube::Schedule.to_s()``` method. Essentially, it adds information on the timeframe when a recurring schedule is relevant. For example, rather than stating that a schedule is "Weekly," ```to_s()``` would output "Weekly, starting on December 12th 2012."

Please let me know if you have any comments on this (especially on the style, etc.) or if this isn't the type of change that you're looking for. Also, thanks for maintaining this library, it's really helpful!

Best,
Kevin