text: Title: [ENH] Safe arithmetic transforms
  Body: What kind of bugs me is that there is no pd.Series.log(), pd.Series.exp(), etc. function that chains the command. To make matters worse, if your Series has zeros, np.log on the Series would return -inf for the zero entries, which screws up any downstream computation --- raising a warning while returning np.nan would be a saner alternative. 
  
  Proposal: add simple chain-able operations to pd.Series like
  - .log(warn=True)
  - .exp
  - .logit(warn=True) [logit(x) = log(x/(1-x)]
  - .sigmoid() [inverse logit]
  - .probit(warn=True) [Phi^-1, where Phi is the standard normal CDF]
  - .normal_cdf() [inverse of probit]
  - .z_score() [standardize by centering and shrinking by standard deviation]
  