ERR: Timestamp.__new__ accepts both tz and tzinfo
Why are there separate tz and tzinfo kwargs?  The following is not intuitive:

```
In [12]: pd.Timestamp('2017-09-25 21:23', tz='US/Eastern')
Out[12]: Timestamp('2017-09-25 21:23:00-0400', tz='US/Eastern')

In [13]: pd.Timestamp('2017-09-25 21:23', tzinfo='US/Eastern')
Out[13]: Timestamp('2017-09-25 21:23:00')
```
