Review and document the Stats module.

I have not done any major changes in the implementation, so this checkin
mainly just adds documentation to the functions etc. (I will add separate
documentation page in an upcoming commit).

The notable changes are:

 * I removed `Array.copy` from `movingWindowFn`. This function is internal
   and none of our uses mutates the array, so this is safe (it speeds things
   up only a little though). I also made some functions `inline`, but this
   also does not add that much.

 * In `initSumsSparse`, I replaces `Seq.filter` and `Seq.map` with single
   `Array.choose`. This actually makes things a bit faster

 * When calculating expanding variance and standard deviation, I changed
   the code to return `nan` when there are less than 2 elements (before, this
   returned 0 for no elements and `nan` for 1 elements, which, I think,
   does not make sense).

 * I added "normal" versions of all the functions so that you can use e.g.
   `Stat.mean` to get mean of a series. I think we should move all statistics
   into the `Stat` module (for consistency and to cleanup the namespaces).
Merge pull request #180 from tpetricek/GH161-rebase

Review and document the Stats module.

This looks great, will merge!