title: Count all the NA values in one column of a dataframe
body: <p>I looking to generate a table that takes a dataframe, counts all the NA values in each column then returns another dataframe that displays that count. I'm preferring to use dplyr tools here. I've gotten this far:</p>
  
  <pre><code>library(dplyr)
  airquality %&gt;% 
    group_by(Month) %&gt;%
    summarise_each(funs(sum(. == 41, na.rm = TRUE)))
  </code></pre>
  
  <p>This returns a table that counts all the 41's. But if I modify it to NA's like so:</p>
  
  <pre><code>airquality %&gt;% 
    group_by(Month) %&gt;%
    summarise_each(funs(sum(. == "NA")))
  </code></pre>
  
  <p>This doesn't produce the desired output (described above). Any thoughts on how I can generate a table that counts all the NA values in each column?</p>
  
  <p>Thanks in advance</p>