Bug: In pandas.util.testing.assert_almost_equal with regards to datetime64 values
#### Code Sample, a copy-pasteable example if possible

``` python
from dateutil.parser import parse
from datetime import datetime
import numpy as np
import pandas as pd
import pandas.util.testing

test_data = pd.DataFrame({
    '_18_months_eo': np.array([0.05, 0.06, 0.12, 0.15], dtype='float64'),
    '_pct_dlq': np.array([0.432, 1.352, 53.23, 0.532], dtype='float64'),
    '_dollar_spent': np.array([1000.00, -1000.00, 15000.00, 0.01], dtype='float64'),
    '_borrower_name': np.array(['John', 'Mary', 'Sue', 'Nathan'], dtype='object'),
    '_origination_date': np.array(['2015-04-04', pd.NaT, '2008-03-03', 'Mar 2014'], dtype='object')
})

i = pd.to_datetime(datetime.date(parse('Mar 2014')))
print(i)
test_data['_origination_date'][3] = i

expected_result = pd.DataFrame({
    '_borrower_name': np.array(['John', 'Mary', 'Sue', 'Nathan'], dtype='object'),
    '_dollar_spent': np.array([1000.00, -1000.00, 15000.00, 0.01], dtype='float64'),
    '_pct_dlq': np.array([0.432, 1.352, 53.23, 0.532], dtype='float64'),
    '_18_months_eo': np.array([0.05, 0.06, 0.12, 0.15], dtype='float64'),
    '_origination_date': pd.Series(['2015-04-04', pd.NaT, '2008-03-03', '2014-03-01'], dtype='datetime64[ns]')
})

expected_result = expected_result.reindex_axis(sorted(expected_result.columns), axis=1)

test_data = test_data.reindex_axis(sorted(test_data.columns), axis=1)

pd.util.testing.assert_frame_equal(expected_result, test_data)
```

The Output:

``` python
2014-03-02 00:00:00
/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/ipykernel/__main__.py:18: SettingWithCopyWarning: 
A value is trying to be set on a copy of a slice from a DataFrame

See the caveats in the documentation: http://pandas.pydata.org/pandas-docs/stable/indexing.html#indexing-view-versus-copy
---------------------------------------------------------------------------
AssertionError                            Traceback (most recent call last)
<ipython-input-17-9ddff7b0ad9c> in <module>()
     30 test_data = test_data.reindex_axis(sorted(test_data.columns), axis=1)
     31 
---> 32 pd.util.testing.assert_frame_equal(expected_result, test_data)

/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/pandas/util/testing.pyc in assert_frame_equal(left, right, check_dtype, check_index_type, check_column_type, check_frame_type, check_less_precise, check_names, by_blocks, check_exact, check_datetimelike_compat, obj)
   1047                                 check_names=check_names,
   1048                                 check_datetimelike_compat=check_datetimelike_compat,
-> 1049                                 obj='DataFrame.iloc[:, {0}]'.format(i))
   1050 
   1051 

/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/pandas/util/testing.pyc in assert_series_equal(left, right, check_dtype, check_index_type, check_series_type, check_less_precise, check_names, check_exact, check_datetimelike_compat, obj)
    944     else:
    945         assert_almost_equal(left.get_values(), right.get_values(),
--> 946                             check_less_precise, obj='{0}'.format(obj))
    947 
    948     # metadata comparison

pandas/src/testing.pyx in pandas._testing.assert_almost_equal (pandas/src/testing.c:3809)()

pandas/src/testing.pyx in pandas._testing.assert_almost_equal (pandas/src/testing.c:2685)()

/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/pandas/util/testing.pyc in raise_assert_detail(obj, message, left, right)
    817 [left]:  {2}
    818 [right]: {3}""".format(obj, message, left, right)
--> 819     raise AssertionError(msg)
    820 
    821 

AssertionError: DataFrame.iloc[:, 3] are different

DataFrame.iloc[:, 3] values are different (25.0 %)
[left]:  [2015-04-03T20:00:00.000000000-0400, NaT, 2008-03-02T19:00:00.000000000-0500, 2014-02-28T19:00:00.000000000-0500]
[right]: [2015-04-03T20:00:00.000000000-0400, NaT, 2008-03-02T19:00:00.000000000-0500, 2014-03-01T19:00:00.000000000-0500]
```
#### Expected Output

Assertion returns 'True' if one were to change expected_result['_origination_date'][3] to '2014-03-02'.
However, the value of '2014-02-28' in the assertion is concerning given that the values in the DataFrame at that position is '2014-03-02 00:00:00.
#### output of `pd.show_versions()`
## INSTALLED VERSIONS

commit: None
python: 2.7.11.final.0
python-bits: 64
OS: Darwin
OS-release: 15.3.0
machine: x86_64
processor: i386
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8

pandas: 0.17.1
nose: None
pip: 8.0.2
setuptools: 20.0
Cython: 0.23.4
numpy: 1.10.2
scipy: 0.14.0
statsmodels: None
IPython: 4.1.1
sphinx: None
patsy: 0.3.0
dateutil: 2.3
pytz: 2014.10
blosc: None
bottleneck: None
tables: None
numexpr: 2.4.6
matplotlib: None
openpyxl: None
xlrd: 0.9.4
xlwt: None
xlsxwriter: None
lxml: None
bs4: None
html5lib: None
httplib2: None
apiclient: None
sqlalchemy: None
pymysql: None
psycopg2: 2.6.1 (dt dec pq3 ext lo64)
Jinja2: None
