[WiP] Cleaning up statistics functions etc.

Work-in-progress. I would like to do a few more things, discussed in: https://github.com/BlueMountainCapital/Deedle/issues/185

 * Remove dead code in Common.fs and also delete old file Functions.fs
 * Implement our own version of Median and remove Math.NET dependency
   (Math.NET is still great and we should use it for more advanced things,
   but this is better done through extensions in FsLab)
 * Change "Stats" to a type with overloaded methods (details explained
   in https://github.com/BlueMountainCapital/Deedle/issues/185)