Empty value handling in colander

1) For a better compatibility with backend modules or data storage systems it was much more convenient to allow "None" and empty strings for Tuple, Number, Date, DateTime, Time nodes.
The current version raises an Invalid exception on serialization. 

2) Parse Date, DateTime and Time strings in serialize(). Again for a better compatibility.
For example SQLite reads dates/datetimes as strings by default.

Maybe this is not really required for standalone colander usage. But it definetly helps to make deform integration easier.
Though, I don't think this will break anything. Maybe you have a look.

Arndt.