Serialize numpy scalars in pd.msgpack
#### Code Sample, a copy-pastable example if possible

``` python
import numpy as np
import pandas as pd
pd.msgpack.dumps(np.int32(0))
```

Currently procduces:

``` python
TypeError: can't serialize 0
```
#### Expected Output

```
b'\x00'
```
#### output of `pd.show_versions()`

``` python
In [5]: pd.show_versions()

INSTALLED VERSIONS
------------------
commit: None
python: 3.4.4.final.0
python-bits: 64
OS: Linux
OS-release: 3.13.0-59-generic
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8

pandas: 0.17.0rc1+752.gfe201a2
nose: 1.3.7
pip: 8.0.3
setuptools: 20.1.1
Cython: 0.22.1
numpy: 1.10.4
scipy: 0.15.1
statsmodels: 0.6.1
IPython: 4.0.1
sphinx: 1.3.1
patsy: 0.3.0
dateutil: 2.4.2
pytz: 2015.4
blosc: 1.2.8
bottleneck: 1.0.0
tables: 3.2.0
numexpr: 2.4.3
matplotlib: 1.4.3
openpyxl: 1.8.5
xlrd: 0.9.3
xlwt: 1.0.0
xlsxwriter: 0.7.3
lxml: 3.4.4
bs4: 4.3.2
html5lib: None
httplib2: None
apiclient: None
sqlalchemy: 1.0.5
pymysql: None
psycopg2: None
jinja2: 2.8
```

Related to https://github.com/msgpack/msgpack-python/issues/61
