title: Applying the python-geohash encode function on a dataframe
body: <p>I am trying to apply the encode function to a dataframe. I keep meeting a ValueError:</p>
  
  <pre><code>&gt;&gt;&gt; import pandas as pd
  &gt;&gt;&gt; import pygeohash as gh
  &gt;&gt;&gt; data = { 'latitude': [4.123, 24.345, 31.654],  'longitude': [25.432, 4.234, 57.098]}
  &gt;&gt;&gt; df = pd.DataFrame(data)
  &gt;&gt;&gt; df
     latitude  longitude
  0     4.123     25.432
  1    24.345      4.234
  2    31.654     57.098
  &gt;&gt;&gt; df['geohash']=df.apply(lambda x: gh.encode(df.latitude, df.longitude, precision=5), axis=1)
  
  Traceback (most recent call last):
    .........
  ValueError: ('The truth value of a Series is ambiguous. Use a.empty, a.bool(), a.item(), a.any() or a.all().', 'occurred at index 0')
  &gt;&gt;&gt; 
  </code></pre>
  
  <p>Putting in a single pair of values:</p>
  
  <pre><code>&gt;&gt;&gt; gh.encode(22,36, precision = 5)
  'sgct5'
  </code></pre>
  
  <p>shows that gh.encode is working.</p>
  
  <p>Is there any other way of doing this?</p>