Series.argmax and Series.idxmax ignore numpy.inf values
#### A small, complete example of the issue

```python
>>> series = pd.Series({'a':3, 'b':7, 'c':np.inf, 'd':6})
>>> series.argmax() # should be c
'b'
>>> series.idxmax() # should be c
'b'
>>> series.max() # this works properly
inf
>>> series.index[np.argmax(np.array(series))] # this work-around works
'c'
```

#### Expected Output

```python
>>> series = pd.Series({'a':3, 'b':7, 'c':np.inf, 'd':6})
>>> series.argmax()
'c'
>>> series.idxmax()
'c'
```

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

<details>
# Paste the output here pd.show_versions() here
```python
>>> pd.show_versions()

INSTALLED VERSIONS
------------------
commit: None
python: 3.5.2.final.0
python-bits: 64
OS: Darwin
OS-release: 15.6.0
machine: x86_64
processor: i386
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8

pandas: 0.19.0
nose: 1.3.7
pip: 8.1.2
setuptools: 23.0.0
Cython: 0.24
numpy: 1.11.2
scipy: 0.18.1
statsmodels: 0.6.1
xarray: None
IPython: 4.2.0
sphinx: 1.4.1
patsy: 0.4.1
dateutil: 2.5.3
pytz: 2016.7
blosc: None
bottleneck: 1.1.0
tables: 3.2.2
numexpr: 2.6.0
matplotlib: 1.5.1
openpyxl: 2.3.2
xlrd: 1.0.0
xlwt: 1.1.2
xlsxwriter: 0.9.2
lxml: 3.6.0
bs4: 4.4.1
html5lib: None
httplib2: 0.9.2
apiclient: None
sqlalchemy: 1.0.13
pymysql: None
psycopg2: 2.6.2 (dt dec pq3 ext lo64)
jinja2: 2.8
boto: 2.40.0
pandas_datareader: None
```
</details>
