Pandas.to_numeric raises ValueError when applied to python Decimal object
When pandas tries to convert columns to numeric using to_numeric a value error is raised

Example

```python
import decimal
df = pd.DataFrame(dict(col=[decimal.Decimal(9.1)]))
df['col'] = df['col'].apply(pd.to_numeric)
```
however

```python
import decimal
df = pd.DataFrame(dict(col=[decimal.Decimal(9.1)]))
df['col'] = df['col'].astype(float)
```
works fine
#### Problem description

to_numeric should be able to convert decimal objects to float types

#### Output of ``pd.show_versions()``

<details>
INSTALLED VERSIONS
------------------
commit: None
python: 2.7.12.final.0
python-bits: 64
OS: Linux
OS-release: 4.4.0-53-generic
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_IE.UTF-8
LOCALE: None.None

pandas: 0.19.1
nose: 1.3.7
pip: 9.0.1
setuptools: 28.3.0
Cython: 0.23.4
numpy: 1.11.2
scipy: 0.16.1
statsmodels: 0.6.1
xarray: None
IPython: None
sphinx: 1.3.1
patsy: 0.4.1
dateutil: 2.6.0
pytz: 2016.10
blosc: None
bottleneck: None
tables: 3.2.0
numexpr: 2.4.6
matplotlib: 1.5.0
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml: None
bs4: None
html5lib: None
httplib2: 0.9.2
apiclient: None
sqlalchemy: 1.0.9
pymysql: None
psycopg2: None
jinja2: 2.8
boto: 2.38.0
pandas_datareader: None
</details>
