Refactors Alchemy.mount_point

* Its now a class (Alchemy::MountPoint) and not a module anymore.
* It does not raises an error anymore if the mount point could not be found. It returns a default (/) instead.
* Adds specs and raises overall code quality.
Refactors Upgrader to include all older tasks since v2.0

Splitted upgrade tasks in modules of alchemy versions.
Adds a ToDoList module.

Used in the Seeder and the Upgrader.
Renamed module from ToDoList into Shell.
Make 2.0 upgrader module database agnostic.