[WiP] More cleanup and some bug fixes

 * This includes various documentation fixes and improvements. I also updated F# Formatting, so that it now generates nicer Reference API docs with links to GitHub. This is quite useful to get nice overview of the function naming. 

 * I fixed the bug #192, although not by changing `expandCols`, but by fixing `stack` (which now also works better with R, because it creates correctly typed vector).

 * I finished cleanup in the `Stats` module (added back `level` functions and also moved `minBy` and `maxBy` there). This hopefully closes #185

 * I did the renaming discussed in #179, so "union" and "append" functions on series are now both called "merge".

There are two TODO things before we can merge this:

 * I need to fix and update all unit tests
 * I did some more renaming related to "series" and "columns" - I'll post a comment in the discussion for #189 to get more feedback about this.