Implements cross class skills

This closes #10 by making it possible to select cross class skills for character classes.

If a CharacterClass has a skill as a cross class, when a user assigns that skill to a character, it will be assigned as a cross class skill. Otherwise it will be assigned as a regular skill.

This patch also adds some javascript, which ensures a good user experience when selecting the feats. The javascript ensures that the user doesn't use to many skill points or to few. It also displays cross-class skills as half-ranks.