to_json raises 'Unhandled numpy dtype 15' with complex values.
#### Code Sample, a copy-pastable example if possible

import pandas
df = pandas.DataFrame({'a': [complex(3, -1)]})
df.to_json()
#### Expected Output

'{"a":{"0":3-1j}}'
#### Actual output:

```
---------------------------------------------------------------------------
RuntimeError                              Traceback (most recent call last)
<ipython-input-3-b53888fd1707> in <module>()
----> 1 df.to_json()

.../lib/python2.7/site-packages/pandas/core/generic.pyc in to_json(self, path_or_buf, orient, date_format, double_precision, force_ascii, date_unit, default_handler)
    892             force_ascii=force_ascii,
    893             date_unit=date_unit,
--> 894             default_handler=default_handler)
    895 
    896     def to_hdf(self, path_or_buf, key, **kwargs):

.../lib/python2.7/site-packages/pandas/io/json.pyc in to_json(path_or_buf, obj, orient, date_format, double_precision, force_ascii, date_unit, default_handler)
     33             obj, orient=orient, date_format=date_format,
     34             double_precision=double_precision, ensure_ascii=force_ascii,
---> 35             date_unit=date_unit, default_handler=default_handler).write()
     36     else:
     37         raise NotImplementedError("'obj' should be a Series or a DataFrame")

.../lib/python2.7/site-packages/pandas/io/json.pyc in write(self)
     76             date_unit=self.date_unit,
     77             iso_dates=self.date_format == 'iso',
---> 78             default_handler=self.default_handler)
     79 
     80 

RuntimeError: Unhandled numpy dtype 15
```
#### output of `pd.show_versions()`

```
INSTALLED VERSIONS
------------------
commit: None
python: 2.7.11.final.0
python-bits: 64
OS: Darwin
OS-release: 15.0.0
machine: x86_64
processor: i386
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8

pandas: 0.17.1
nose: 1.3.7
pip: 8.0.3
setuptools: 19.7
Cython: 0.23.4
numpy: 1.8.2
scipy: 0.15.1
statsmodels: 0.6.1
IPython: 4.0.1
sphinx: 1.3.3
patsy: 0.4.1
dateutil: 2.4.2
pytz: 2015.7
blosc: None
bottleneck: None
tables: None
numexpr: None
matplotlib: 1.4.3
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml: None
bs4: None
html5lib: None
httplib2: None
apiclient: None
sqlalchemy: None
pymysql: None
psycopg2: None
Jinja2: None
```
