read_csv: Tz-aware datetime.datetime cannot be converted to datetime64

This may be related to some of the other TimeZone issues.

This worked in 0.8, but now raises an error in 0.8.1:

```
import pandas
import StringIO

data = StringIO.StringIO("Date,x\n2012-06-13T01:39:00Z,0.5")
pandas.read_csv(data,index_col=0, parse_dates=True)
```

Error:

```
ValueError: Tz-aware datetime.datetime cannot be converted to datetime64 unless utc=True
```
