A less evil hack

With this, DescriptiveStatistics module is really included.

On Rubinius only, this may be significantly slower than the previous version
https://github.com/rubinius/rubinius/issues/2390.
Restructure

* Move all calculation methods into Calculation
* Make all of them external to the collection (remove default value from
  the argument
get rid of empty collection methods
better sum
remove #number in favour of standard Ruby size
Remove Gemfile.lock
Avoid modifying input unless it's a hash

This means preserving types when possible, instead of converting
everything to_f
Readme for the fork [ci skip]