Series.idxmax() and Series.max() show inconsistent behaviour with pd.np.Inf
`idxmax` is supposed to return the index of the maximum element. However, it seems to ignore special values like infinity (`pd.np.Inf`), leading to the following inconsistency:

``` python
>>> from pandas import DataFrame
>>> s = pd.Series([0,5,pd.np.Inf])
>>> s.max() == s.ix[s.idxmax()]
```

This snippet will return `False`, since `s.idxmax()` will return `1`, the index of value `5`, while `s.max()` returns `Inf` as expected.
#### Expected Output

``` python
True
```
#### output of `pd.show_versions()`

```
INSTALLED VERSIONS
------------------
commit: None
python: 2.7.12.final.0
python-bits: 64
OS: Linux
OS-release: 4.6.4-1-ARCH
machine: x86_64
processor: 
byteorder: little
LC_ALL: None
LANG: de_DE.UTF-8

pandas: 0.18.1
nose: 1.3.6
pip: 8.1.2
setuptools: 25.1.0
Cython: None
numpy: 1.11.1
scipy: 0.14.0
statsmodels: 0.6.1
xarray: None
IPython: 5.0.0
sphinx: None
patsy: 0.4.1
dateutil: 2.4.2
pytz: 2015.2
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: 2.6.2 (dt dec pq3 ext lo64)
jinja2: 2.8
boto: None
pandas_datareader: None
```
