Implement calling methods of System.Convert with objects

Some minor improvements to the System.Convert class:
* Convert.ChangeType now correctly checks if the object implements IConvertible
* Convert.ToString(obj) calls obj.toString()
* Convert.To____(null) returns 0, false or "" depending on the method
* Convert.To____(obj) checks if the object implements IConvertible, is so, calls it
* Convert.ToChar({int, uint, long, ulong})

It should also support enums once #211 is resolved.