title: Pandas slicing/selecting with multiple conditions with or statement
body: <p>When I select by chaining different conditions with "AND" the selection works fine. When I select by chaining conditions with "OR" the selection throws an error.</p>
  
  <pre><code>&gt;&gt;&gt; import pandas as pd
  &gt;&gt;&gt; import numpy as np
  &gt;&gt;&gt; df = pd.DataFrame([[1,4,3],[2,3,5],[4,5,6],[3,2,5]], 
  ...     columns=['a', 'b', 'c'])
  &gt;&gt;&gt; df
     a  b  c
  0  1  4  3
  1  2  3  5
  2  4  5  6
  3  3  2  5
  &gt;&gt;&gt; df.loc[(df.a != 1) &amp; (df.b &lt; 5)]
     a  b  c
  1  2  3  5
  3  3  2  5
  &gt;&gt;&gt; df.loc[(df.a != 1) or (df.b &lt; 5)]
  Traceback (most recent call last):
    File "&lt;stdin&gt;", line 1, in &lt;module&gt;
    File "/usr/lib/python3/dist-packages/pandas/core/generic.py", line 731, in __nonzero__
      .format(self.__class__.__name__))
  ValueError: The truth value of a Series is ambiguous. Use a.empty, a.bool(), a.item(), a.any() or a.all().
  </code></pre>
  
  <p>I would expect it to return the whole dataframe as all rows meet this condition.</p>