BUG: .iloc does not accept boolean series
#### Code Sample, a copy-pastable example if possible

```python
>>> s = pd.Series([3,4,5])
>>> b = s > 3
>>> b  # a boolean series
0    False
1     True
2     True
dtype: bool
```

Now, various indexing method with ``b``:

```python
>>> s.loc[b]  # ok
>>> s[b]  # ok
>>> s.iloc[b.values]  # ok
>>>s.iloc[b]  # not ok
NotImplementedError: iLocation based boolean indexing on an integer type is not available
```

#### Problem description

As the indexes of ``s`` and ``b`` are the same and the ``b. values`` are booleans, ``.iloc`` should work here (unless I'm missing something non-obvious). That is, it should join up the indices as per the panda rules, and then do a boolean filter.

I did some poking around the error frames, and the start function is ``pandas.core.indexing._iLocIndexer._getitem_axis``.  The error appears to be that ``pandas.core.indexing._iLocIndexer._has_valid_type(key, axis)`` check function is overzealous. If we call ``pandas.core.indexing._iLocIndexer._getbool_axis(key, axis=axis)`` without doing the check in ``._has_valid_type``, everything comes out fine.

It seems to me that ``pandas.core.indexing._iLocIndexer._has_valid_type`` is too strict with boolean series. Is this correct?

#### Expected Output
``s`` filtered by ``b``.

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

<details>

[paste the output of ``pd.show_versions()`` here below this line]
INSTALLED VERSIONS
------------------
commit: f20721940e7be5d741d158fa3e9272b6cbbaa3b5
python: 3.6.2.final.0
python-bits: 64
OS: Windows
OS-release: 10
machine: AMD64
processor: Intel64 Family 6 Model 78 Stepping 3, GenuineIntel
byteorder: little
LC_ALL: None
LANG: None
LOCALE: None.None

pandas: 0.20.3
pytest: 3.2.1
pip: 9.0.1
setuptools: 36.3.0
Cython: 0.25.2
numpy: 1.13.1
scipy: 0.19.1
xarray: None
IPython: 6.1.0
sphinx: 1.6.2
patsy: 0.4.1
dateutil: 2.6.0
pytz: 2017.2
blosc: None
bottleneck: None
tables: None
numexpr: None
feather: None
matplotlib: 2.0.2
openpyxl: 2.4.5
xlrd: 1.0.0
xlwt: None
xlsxwriter: None
lxml: 3.7.3
bs4: 4.5.3
html5lib: 0.999999999
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: 2.9.6
s3fs: None
pandas_gbq: None
pandas_datareader: None
</details>
