Negative timedeltas for deltas > 292 years
#### Code Sample, a copy-pastable example if possible

```
import pandas as pd
dates = pd.Series(pd.date_range('1700-01-01', periods=4500, freq='m'))
days_delta = (dates-dates.min()).astype('timedelta64[D]')
```
#### Expected Output

Should return only positive timedeltas, however for deltas greater than 292 years the values are negative. 
#### output of `pd.show_versions()`

commit: None
python: 3.4.4.final.0
python-bits: 64
OS: Windows
OS-release: 8.1
machine: AMD64
processor: Intel64 Family 6 Model 58 Stepping 9, GenuineIntel
byteorder: little
LC_ALL: None
LANG: None

pandas: 0.17.1
nose: None
pip: 8.0.3
setuptools: 20.1.1
Cython: 0.23.4
numpy: 1.10.4
scipy: 0.17.0
statsmodels: 0.6.1
IPython: 4.0.3
sphinx: 1.3.5
patsy: 0.4.1
dateutil: 2.5.0
pytz: 2015.7
blosc: None
bottleneck: 1.0.0
tables: 3.2.2
numexpr: 2.4.6
matplotlib: 1.5.1
openpyxl: 2.3.2
xlrd: 0.9.4
xlwt: 1.0.0
xlsxwriter: 0.8.4
lxml: 3.5.0
bs4: 4.4.1
html5lib: 0.999
httplib2: None
apiclient: None
sqlalchemy: 1.0.11
pymysql: 0.6.2.None
psycopg2: None
Jinja2: 2.8
