title: Replace string/value in entire DataFrame
body: <p>I have a very large dataset were I want to replace strings with numbers. I would like to operate on the dataset without typing a mapping function for each key (column) in the dataset. (similar to the fillna method, but replace specific string with assosiated value).
  Is there anyway to do this?</p>
  
  <p>Here is an example of my dataset</p>
  
  <pre><code>data
     resp          A          B          C
  0     1       poor       poor       good
  1     2       good       poor       good
  2     3  very good  very good  very good
  3     4       bad        poor       bad 
  4     5   very bad   very bad   very bad
  5     6       poor       good   very bad
  6     7       good       good       good
  7     8  very good  very good  very good
  8     9       bad        bad    very bad
  9    10   very bad   very bad   very bad
  </code></pre>
  
  <p>The desired result:</p>
  
  <pre><code> data
     resp  A  B  C
  0      1  3  3  4
  1     2  4  3  4
  2     3  5  5  5
  3     4  2  3  2
  4     5  1  1  1
  5     6  3  4  1
  6     7  4  4  4
  7     8  5  5  5
  8     9  2  2  1
  9    10  1  1  1
  </code></pre>
  
  <p>very bad=1, bad=2, poor=3, good=4, very good=5</p>
  
  <p>//Jonas</p>