title: Removing NA columns in xts
body: <p>I have an xts in the following format</p>
  
  <pre><code>                   a        b     c        d       e        f   ......
  2011-01-03         11.40    NA    23.12    0.23    123.11   NA  ......
  2011-01-04         11.49    NA    23.15    1.11    111.11   NA  ......
  2011-01-05         NA       NA    23.11    1.23    142.32   NA  ......
  2011-01-06         11.64    NA    39.01    NA      124.21   NA  ......
  2011-01-07         13.84    NA    12.12    1.53    152.12   NA  ......
  </code></pre>
  
  <p>Is there a function I can apply to generate a new xts or data.frame missing the columns containing only NA? </p>
  
  <p>The position of the columns with the NAs isn't static so just removing those columns by name or position isn't possible</p>