Package clean up

Stats.jl is too long and difficult to navigate. I dispatched the
functions into smaller source files.

In addition, I removed several functions that have been provided in
other packages:
- distance: Distance.jl provides a much faster version.
- logit: has been provided in NumericExtensions.jl

Moreover, I fixed midrange, and added minmax, and range.

There are also minor clean up & fixes.