BUG: alignment not happening on in-place ops
#### Code Sample, a copy-pastable example if possible

```
In [2]: a = pd.Series(1, index=range(3))

In [3]: b = pd.Series(2, index=range(4))

In [4]: a.dtype
Out[4]: dtype('int64')

In [5]: a += b

In [6]: a.dtype
Out[6]: dtype('float64')

In [7]: a.index == b.index
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-8-2fae764c9047> in <module>()
----> 1 a.index == b.index

/home/nobackup/repo/pandas/pandas/indexes/base.pyc in _evaluate_compare(self, other)
   3026                 if isinstance(other, (np.ndarray, Index, ABCSeries)):
   3027                     if other.ndim > 0 and len(self) != len(other):
-> 3028                         raise ValueError('Lengths must match to compare')
   3029                 func = getattr(self.values, op)
   3030                 result = func(np.asarray(other))

ValueError: Lengths must match to compare
```
#### Expected Output

My preference would be

```
In [6]: a.dtype
Out[6]: dtype('int64')
```

... but if one prioritises coherence with `a = a + b`, then the expected behaviour would probably be

```
In [6]: a.dtype
Out[6]: dtype('float64')

In [7]: a.index == b.index
Out[7]: array([ True,  True,  True,  True], dtype=bool)
```

(outer join). Instead I think the current behaviour is hard to rationalise (i.e. it is dictated by implementation).
#### output of `pd.show_versions()`

```
In [7]: pd.show_versions()

INSTALLED VERSIONS
------------------
commit: 22cf50bbef7f28c63941328ea47502f1b202bda2
python: 2.7.10.final.0
python-bits: 64
OS: Linux
OS-release: 4.4.0-1-amd64
machine: x86_64
processor: 
byteorder: little
LC_ALL: None
LANG: it_IT.utf8

pandas: 0.18.0+48.g22cf50b.dirty
nose: 1.3.6
pip: 1.5.6
setuptools: 18.4
Cython: 0.23.2
numpy: 1.10.4
scipy: 0.16.0
statsmodels: 0.8.0.dev0+755fa81
xarray: None
IPython: 2.4.1
sphinx: 1.3.1
patsy: 0.3.0-dev
dateutil: 2.2
pytz: 2012c
blosc: None
bottleneck: None
tables: 3.2.2
numexpr: 2.4.3
matplotlib: 1.5.0rc2
openpyxl: 2.3.0
xlrd: 0.9.4
xlwt: 0.7.5
xlsxwriter: 0.7.3
lxml: 3.4.4
bs4: 4.4.0
html5lib: 0.999
httplib2: 0.9.1
apiclient: None
sqlalchemy: 1.0.11
pymysql: None
psycopg2: 2.6.1 (dt dec mx pq3 ext lo64)
jinja2: 2.8
boto: 2.38.0
pandas_datareader: None
```
