title: Remove rows of zeros from a Pandas series
body: <p>I have a number <code>Pandas</code> Series with 601 rows indexed by date as seen below. The values are zero up until a point, after which all the values are non zero. This point varies with each Series but I would like a way to remove all the rows where the value is zero while keeping the integrity of the date index.</p>
  
  <pre><code>Name: users, dtype: float64 dates
  2015-08-17 14:29:59-04:00    18
  2015-08-16 14:29:59-04:00     3
  2015-08-15 14:29:59-04:00    11
  2015-08-14 14:29:59-04:00    12
  2015-08-13 14:29:59-04:00     8
  2015-08-12 14:29:59-04:00    10
  2015-08-11 14:29:59-04:00     6
  2015-08-10 14:29:59-04:00     6
  2015-08-09 14:29:59-04:00     7
  2015-08-08 14:29:59-04:00     7
  2015-08-07 14:29:59-04:00    13
  2015-08-06 14:29:59-04:00    16
  2015-08-05 14:29:59-04:00    12
  2015-08-04 14:29:59-04:00    14
  2015-08-03 14:29:59-04:00     5
  2015-08-02 14:29:59-04:00     5
  2015-08-01 14:29:59-04:00     8
  2015-07-31 14:29:59-04:00     6
  2015-07-30 14:29:59-04:00     7
  2015-07-29 14:29:59-04:00     9
  2015-07-28 14:29:59-04:00     7
  2015-07-27 14:29:59-04:00     5
  2015-07-26 14:29:59-04:00     4
  2015-07-25 14:29:59-04:00     8
  2015-07-24 14:29:59-04:00     8
  2015-07-23 14:29:59-04:00     8
  2015-07-22 14:29:59-04:00     9
  2015-07-21 14:29:59-04:00     5
  2015-07-20 14:29:59-04:00     7
  2015-07-19 14:29:59-04:00     6
                               ..
  2014-01-23 13:29:59-05:00     0
  2014-01-22 13:29:59-05:00     0
  2014-01-21 13:29:59-05:00     0
  2014-01-20 13:29:59-05:00     0
  2014-01-19 13:29:59-05:00     0
  2014-01-18 13:29:59-05:00     0
  2014-01-17 13:29:59-05:00     0
  2014-01-16 13:29:59-05:00     0
  2014-01-15 13:29:59-05:00     0
  2014-01-14 13:29:59-05:00     0
  2014-01-13 13:29:59-05:00     0
  2014-01-12 13:29:59-05:00     0
  2014-01-11 13:29:59-05:00     0
  2014-01-10 13:29:59-05:00     0
  2014-01-09 13:29:59-05:00     0
  2014-01-08 13:29:59-05:00     0
  2014-01-07 13:29:59-05:00     0
  2014-01-06 13:29:59-05:00     0
  2014-01-05 13:29:59-05:00     0
  2014-01-04 13:29:59-05:00     0
  2014-01-03 13:29:59-05:00     0
  2014-01-02 13:29:59-05:00     0
  2014-01-01 13:29:59-05:00     0
  2013-12-31 13:29:59-05:00     0
  2013-12-30 13:29:59-05:00     0
  2013-12-29 13:29:59-05:00     0
  2013-12-28 13:29:59-05:00     0
  2013-12-27 13:29:59-05:00     0
  2013-12-26 13:29:59-05:00     0
  2013-12-25 13:29:59-05:00     0
  </code></pre>