Datetime objects are converted to Timestamp without my consent.
Python datetime.datetime object get converted inside pandas dataframes. This is not in all cases desired. 

#### Code Sample
```from datetime import datetime
import numpy as np
import pandas as pd

rd = lambda : datetime(2017,1,np.random.randint(1,32))
datelist = [rd() for i in range(10)]
print(type(datelist[0])) # <type 'datetime.datetime'>

df = pd.DataFrame({"date" : datelist,
                   "y" : np.random.rand(10)})

print(type(df["date"][0])) # <class 'pandas._libs.tslib.Timestamp'>

dates = [d.to_pydatetime() for d in df["date"]]
print(type(dates[0])) # <type 'datetime.datetime'>
```

Creating a dataframe with a list of `datetime` objects converts them to `pandas._libs.tslib.Timestamp`. This is undesired. In this simple case it is at least possible to get back a datetime object by list comprehension, `dates = [d.to_pydatetime() for d in df["date"]]`, which I would consider as an unnecessary step and makes the use of a DataFrame somehow obsolete.

Background:

The real problem with this behaviour is shown in the following. Any instance of a class that gets derived from `datetime` is also converted, hence loosing all its properties.

```from datetime import datetime
import numpy as np
import pandas as pd

class someobject(datetime):
    prop = "property"
    def __init__(self,*args,**kwargs):
        datetime.__init__(*args,**kwargs)

rd = lambda : someobject(2017,1,np.random.randint(1,32))
datelist = [rd() for i in range(10)]
print(type(datelist[0])) # <class '__main__.someobject'>
print(datelist[0].prop)  # property

df = pd.DataFrame({"date" : datelist,
                   "y" : np.random.rand(10)})

print(type(df["date"][0])) # <class 'pandas._libs.tslib.Timestamp'>
print(df["date"][0].prop)  # Error AttributeError: 'Timestamp' object has no attribute 'prop'
```

How to prevent the DataFrame to convert an object that I put into it to something else?  
As a workaround, how to change `someobject` in the above such that it does not get converted automatically to something else?

[Here, python 2.7, pandas 0.20.1 are used]