modernize syntax in Timestamp
Small cleanups to `Timestamp`+`_Timestamp`, use `@property` instead of older syntax.

The only substantive change is in `__add__`, where `result = Timestamp(normalize_date(result))` is changed to `result = result.normalize()`.  The latter has the benefits
1) is robust for tzaware timestamps,

2) does not require `normalize_date` in the namespace, so after this `Timestamp`+`_Timestamp` can be cut/paste into a new `tslibs.timestamps` and

3) is robust to subclassing of `Timestamp`.