Add log return version of pct_change
`NDFrame` has a `pct_change` method that works as: `x_t/x_{t-period} - 1`

`log_return` would work as: `log(x_t/x_{t-period})`

These two values are approximately equal for small values of `x_t/x_{t-period}`.

The log return version tends to be used in finance more often than the pct_change.
