BUG: provide for automatic conversion of object -> datetime64
- construct series that have M8[ns] dtype even in the presence of NaN (rather than object)
  see below. 
- allow np.nan to be used in place of NaT in series construction (requires explicity dtype to be passed)
  e.g. `Series(np.nan,index=range(5),dtype='M8[ns]')`
- bug fix in applymap (tseries/tests/test_timeseries/test_frame_apply_dont_convert_datetime64), because explicit
  conversion of the series to M8ns, now has the function operating on datetime64 values and ufuncs dont' work for some reason,
  explicity required conversion to Timestamp values make this work (same issue as in GH #2591)

```
In [1]: import numpy as np

In [2]: import pandas as pd
```

This currently works

```
In [12]: df = pd.DataFrame(np.random.randn(8,2),pd.date_range('20010102',periods=8),columns=['A','B'])

In [13]: df['timestamp'] = pd.Timestamp('20010103')

In [14]: x = df.convert_objects()

In [15]: x
Out[15]: 
                   A         B           timestamp
2001-01-02 -0.228005 -1.376432 2001-01-03 00:00:00
2001-01-03 -0.555251  0.277356 2001-01-03 00:00:00
2001-01-04 -0.295289  0.534981 2001-01-03 00:00:00
2001-01-05 -0.332815 -1.436975 2001-01-03 00:00:00
2001-01-06 -1.848818  0.405773 2001-01-03 00:00:00
2001-01-07 -0.734903 -0.174140 2001-01-03 00:00:00
2001-01-08 -0.388864 -0.765329 2001-01-03 00:00:00
2001-01-09 -0.341015  0.170630 2001-01-03 00:00:00

In [16]: x.get_dtype_counts()
Out[16]: 
datetime64[ns]    1
float64           2

In [21]: x.ix[3,'timestamp'] = iNaT

In [22]: x
Out[22]: 
                   A         B           timestamp
2001-01-02 -0.228005 -1.376432 2001-01-03 00:00:00
2001-01-03 -0.555251  0.277356 2001-01-03 00:00:00
2001-01-04 -0.295289  0.534981 2001-01-03 00:00:00
2001-01-05 -0.332815 -1.436975                 NaT
2001-01-06 -1.848818  0.405773 2001-01-03 00:00:00
2001-01-07 -0.734903 -0.174140 2001-01-03 00:00:00
2001-01-08 -0.388864 -0.765329 2001-01-03 00:00:00
2001-01-09 -0.341015  0.170630 2001-01-03 00:00:00

```

This leaves you in limbo, as you have basically a datatime64 column, but with a float nan in it (so it has to stay object)

```
In [3]: df = pd.DataFrame(np.random.randn(8,2),pd.date_range('20010102',periods=8),columns=['A','B'])

In [4]: df
Out[4]: 
                   A         B
2001-01-02  0.939393 -2.524448
2001-01-03 -1.059561  0.104651
2001-01-04 -0.842478  1.033888
2001-01-05 -1.009903 -0.334782
2001-01-06 -0.452043 -0.382408
2001-01-07 -0.058516  1.162884
2001-01-08  0.660251  0.688290
2001-01-09  0.069637  0.366915

In [5]: df['timestamp'] = pd.Timestamp('20010103')

In [6]: df
Out[6]: 
                   A         B            timestamp
2001-01-02  0.939393 -2.524448  2001-01-03 00:00:00
2001-01-03 -1.059561  0.104651  2001-01-03 00:00:00
2001-01-04 -0.842478  1.033888  2001-01-03 00:00:00
2001-01-05 -1.009903 -0.334782  2001-01-03 00:00:00
2001-01-06 -0.452043 -0.382408  2001-01-03 00:00:00
2001-01-07 -0.058516  1.162884  2001-01-03 00:00:00
2001-01-08  0.660251  0.688290  2001-01-03 00:00:00
2001-01-09  0.069637  0.366915  2001-01-03 00:00:00

In [7]: df.get_dtype_counts()
Out[7]: 
float64    2
object     1

In [9]: df.ix[3,'timestamp'] = np.nan

In [10]: df
Out[10]: 
                   A         B            timestamp
2001-01-02  0.939393 -2.524448  2001-01-03 00:00:00
2001-01-03 -1.059561  0.104651  2001-01-03 00:00:00
2001-01-04 -0.842478  1.033888  2001-01-03 00:00:00
2001-01-05 -1.009903 -0.334782                  NaN
2001-01-06 -0.452043 -0.382408  2001-01-03 00:00:00
2001-01-07 -0.058516  1.162884  2001-01-03 00:00:00
2001-01-08  0.660251  0.688290  2001-01-03 00:00:00
2001-01-09  0.069637  0.366915  2001-01-03 00:00:00

In [11]: df.convert_objects().get_dtype_counts()
Out[11]: 
float64    2
object     1
```

This PR enables this syntax:

```
In [3]: df = pd.DataFrame(np.random.randn(8,2),pd.date_range('20010102',periods=8),columns=['A','B'])

In [4]: df['timestamp'] = pd.Timestamp('20010103')

# datetime64[ns] out of the box
In [5]: df.get_dtype_counts()
Out[5]: 
datetime64[ns]    1
float64           2

# use the traditional nan, which is mapped to iNaT internally
In [6]: df.ix[2:4,'timestamp'] = np.nan

In [7]: df
Out[7]: 
                   A         B           timestamp
2001-01-02  0.234354 -3.167626 2001-01-03 00:00:00
2001-01-03  0.141300 -0.548670 2001-01-03 00:00:00
2001-01-04  0.738884 -0.104497                 NaT
2001-01-05  0.873046  1.193593                 NaT
2001-01-06  1.174226  1.697421 2001-01-03 00:00:00
2001-01-07 -0.069534 -0.145611 2001-01-03 00:00:00
2001-01-08 -0.951426 -1.158884 2001-01-03 00:00:00
2001-01-09 -0.322041 -0.800181 2001-01-03 00:00:00

```
