Adds Skills

Skills are basically method modifiers. They jump on methods, and
and change their return values. For example, the Farsight skill
increases LOS by two, by intercepting the LOS distance method.

Skills can either used to change return values, or to trigger
callbacks. As proof of concept of that, the Vampirism skill restores
health when a unit calls #hit.

This commit also removes a lot of the basic configuration of classes
in favor of implementing it through skills.
more info in unit info panel
Stop the cursor for flashing in the wrong location
Ban clerics
make curses executable
Load the save file only if it exists, but don't rescue from errors.