to_html() formatters does not work for objects with 'datetime64[ns]' type
I am using a to_html() to convert a dataframe into a table. i have various datatypes in Dataframe including float, int, datetime and timedelta. 

I am using formatters to control outputs, It seems to be working for int and float but not for datetime. 

``` python
datetime_formatter = lambda ts: "%s" % ts.strftime("%I:%M %p")
currency_formatter = lambda cash: "%.2f" % cash
```
