Data standardization: z-score and 0-1 scaling

Interface and implementation of data standardization (normalization) techniques. A proposed interface would allow scale data and reverse it to its original state. Currently available: z-score and 0-1 forward scaling.
These are commonly used scaling techniques in data analysis and machine learning.

The idea originated in MLBase package, but it was [proposed](https://github.com/JuliaStats/MLBase.jl/pull/9) to move the code into StatsBase. So my question is whether this functionality should be integrated in StatsBase or move it into separate package.
This double-allocates memory I believe because `ifelse` evaluates both of its arguments.