title: Remove rows with all or some NAs (missing values) in data.frame
body: <p>I'd like to remove the lines in this data frame that:</p>
  
  <p>a) <strong>contain <code>NA</code>s across all columns.</strong> Below is my example data frame. </p>
  
  <pre><code>             gene hsap mmul mmus rnor cfam
  1 ENSG00000208234    0   NA   NA   NA   NA
  2 ENSG00000199674    0   2    2    2    2
  3 ENSG00000221622    0   NA   NA   NA   NA
  4 ENSG00000207604    0   NA   NA   1    2
  5 ENSG00000207431    0   NA   NA   NA   NA
  6 ENSG00000221312    0   1    2    3    2
  </code></pre>
  
  <p>Basically, I'd like to get a data frame such as the following.</p>
  
  <pre><code>             gene hsap mmul mmus rnor cfam
  2 ENSG00000199674    0   2    2    2    2
  6 ENSG00000221312    0   1    2    3    2
  </code></pre>
  
  <p>b) <strong>contain <code>NA</code>s in only some columns</strong>, so I can also get this result:</p>
  
  <pre><code>             gene hsap mmul mmus rnor cfam
  2 ENSG00000199674    0   2    2    2    2
  4 ENSG00000207604    0   NA   NA   1    2
  6 ENSG00000221312    0   1    2    3    2
  </code></pre>