title: Time efficient way for add leading zeros in pandas series
body: <p>I'm a beginner in python. I'm using python 3.X in my windows 7
  I have a pandas series. Currently it contains 1544252 records of 8 digit number of int64 type. Sample of last few records are given below</p>
  
  <pre><code>1544241    87526878
  1544242    92480921
  1544243    61479211
  1544244    57572826
  1544245    25527504
  1544246    69477558
  1544247    35477532
  1544248    12475315
  1544249    75480854
  1544250    68527476
  1544251    87480808
  </code></pre>
  
  <p>I need to add leading zeros in my series to make 8 digit number to 10 character long string. Here is my desired output.</p>
  
  <pre><code>    1544241    0087526878
      1544242    0092480921
      1544243    0061479211
      1544244    0057572826
      1544245    0025527504
      1544246    0069477558
      1544247    0035477532
      1544248    0012475315
      1544249    0075480854
      1544250    0068527476
      1544251    0087480808
  </code></pre>
  
  <p>Can you please suggest me how to do that in time efficient way?</p>
  
  <p>Thanks in advance</p>