Timestamp("now") should match to_datetime("now")
See #17697, #18666

`pd.to_datetime('now')` returns the current UTC time, while `pd.Timestamp('now')` returns the current local time.  `pd.to_datetime('today')` rounds the current UTC time down to midnight, while `pd.Timestamp('today')` returns the current local time (not rounded down).  These should be fixed to follow a single convention.

```
>>> pd.Timestamp('now')
Timestamp('2017-12-09 12:28:31.642416')
>>> pd.to_datetime('now')
Timestamp('2017-12-09 20:28:35')
>>> pd.Timestamp('today')
Timestamp('2017-12-09 12:28:49.263407')
>>> pd.to_datetime('today')
Timestamp('2017-12-09 00:00:00')
```

AFAICT the to_datetime behavior was designed to match the behavior of `np.datetime64('now')` and `np.datetime64('today')`.  The Timestamp behavior matches the special constructors `Timestamp.now()`, `Timestamp.today()`.  At this point I think the least disruptive change would be to make Timestamp(...) behave like to_datetime(...).

Thoughts?