Errors in castTo() for numeric and date types

[davethecanuck]

TIBRVMSG_I8 handling references self.data.i8.value but should instead just reference self.data.i8. The 'value' attribute is not valid as the value is stored at the 'i8' level. This same error exists in the other signed and unsigned numeric types. Removing the '.value' suffix fixes the issue.

Likewise, the DATETIME handler doesn't work as the 'value' attribute does not exist for the date.sec and date.nsec fields. This type handler has an additional problem as 'obj.data=dt' fails with: "AttributeError: can't set attribute".