title: how to add add list element names as a variable in the output
body: <p>I have a list of data.frames looks like this:</p>
  
  <pre><code>$IB1Q
           V1          V2          V3          V4          V5          V6          V7          V8 
  0.184011746 0.805070251 0.840738769 0.701748191 0.493042421 0.889981450 0.003934755 0.794561335 
           V9         V10         V11 
  0.664718057 0.880357401 0.545795252 
  
  $IBCA
         V1        V2        V3        V4        V5        V6        V7        V8        V9 
  0.8719393 0.8177656 0.9205127 0.7200678 0.7456413 0.4643883 0.8006192 0.1430972 0.7622733 
        V10       V11 
  0.4716331 0.8681162 
  </code></pre>
  
  <p>and each element name (IB1Q, IBCA) conveys information. I want to row.bind them together and in my data.farme I want a 
  variable for the list elemnt each observation originated in.
  So I want to get a data.frame with 2 columns (first column popID and second columns my proportions):</p>
  
  <pre><code>IB1Q   0.184011746
  IB1Q   0.805070251
  IB1Q   0.840738769
  IB1Q   0.701748191
  IB1Q   0.493042421
  .
  .
  .
  IBCA   0.4716331
  IBCA   0.8681162
  </code></pre>