Make Timestamp('now') equivalent to Timestamp.now()

closes #9000 
I opted to not change np_datetime_strings but instead short-circuit in Timestamp.  I don't have a strong preference one way or another.
In the standard library `today() = now()` for datetimes.  Is that what we want here or do we want the `datetime.date.today()` behavior?

```
datetime.date.today()
Out[15]: datetime.date(2014, 12, 6)
datetime.datetime.now()
Out[16]: datetime.datetime(2014, 12, 6, 14, 20, 26, 33218)
datetime.datetime.today()
Out[17]: datetime.datetime(2014, 12, 6, 14, 20, 30, 519572)
pandas.Timestamp('today')
Out[20]: Timestamp('2014-12-06 00:00:00')
```